I believe you're referring to the Perry expedition? It was a little bit different than the trading you seem to be referring to. It was a diplomatic mission, not a trade one. While they did engage in a limited amount of trade while there, Perry's primary mission was just to open the country, with the real trade coming later.

The Americans themselves thought they were being fair with the Japanese and weren't asking anything egregious of them. Their final takeaway was opening two ports to American ships, establishing an American consulate, making provisos for the return of shipwrecked sailors, and building a coaling station.

That being said, their methods were certainly heavy-handed. The expedition had many modern military ships, a full compliment of marines, and was a diplomatic mission conducted entirely by the military, with no diplomats involved.

There were reasons for this; the US didn't come with warships just for purposes of blackmail. There had been multiple visits and attempts at opening Japan before Perry that had mostly gone poorly. In 1837 US businessman Charles King attempted to return some Japanese maroons and enter negotiations for trade while there. The ship was attacked by the Japanese and had to depart without even being able to return the shipwrecked Japanese. In 1849 the US learned of the presence of American maroons in Japan from the Dutch, and Captain James Glynn sailed to Nagasaki to demand their return. It was his recommendation afterwards that led to the decision to accompany future negotiations with a show of force.

Now, as to the heart of the question, did the Japanese get a "fair deal"? This is a pretty subjective question. In the long run they almost certainly did not. Japan was subjected to unequal treaties like many eastern countries were, putting them in a disadvantageous situation in negotiations and providing extraterritoriality for foreigners. But most of those came after, starting with the 1858 Harris Treaty.

Was the 1854 Convention of Kanagawa signed by Perry itself an unequal treaty? Looking at the twelve articles most of it is not so exploitative, with the exception of the Article 9 most favored nation clause.

As for the negotiations themselves, the Americans probably thought they were just being firm with a backwards regime that did all in its power to refuse to acknowledge other nations, but the Japanese were in a panic. In practice the US delegation had an attitude of "our way or the highway". Whenever they thought the Japanese were trying to give them the runaround, dismiss them, or giving them a bad deal, they would make a threat. When the local Japanese governor refused to accept their message on Perry's first visit and asked them to go to the Dutch port of Dejima at Nagasaki, Perry threatened to land near Edo with his marines and march to deliver the letter to the Shogun himself. It's those sorts of interactions, where the Americans thought they were being subjected to undue suspicion and not being given the time of day, and the Japanese were terrified over the American's threats.

That being said, extensive negotiations did take place over several months, and the Americans didn't get quite all that they wanted. But the threat of force was a trump card they could use to keep the negotiations open when the Japanese would have liked to close or downgrade them. And more to the point, the memory of that threat of force remained afterwards, and was partly responsible for the later unequal treaties, the Meiji restoration, and Japan's drive to modernize and expand their military.

Personally, while the US definitely used force to get their way in negotiations in a way that could be considered blackmail, and even though they were forcibly imposing their will on another nation's desire to remain isolated, I don't think the Convention of Kanagawa itself was that harsh, all things considered. The Tokugawa Shogunate's desire to still remain closed to all foreign interaction in the late 19th century was a pipe dream. The real abuses of the unequal treaties wouldn't come until later.

Sources:

Williams, Samuel Wells, A journal of the Perry expedition to Japan (1853-1854), 1910

One of the more pernicious articles of the Convention of Kanagawa was Article VII on the exchange of currency between Japan and foreign traders. The currency system of the Shogunate was somewhat heterogeneous and it made it difficult for the foreign merchants to deal in Japan. During the subsequent negotiations with the US, Townsend Harris insisted that Japanese silver currency be pegged to the Mexican silver dollar by weight as a means for foreign traders to do business in the islands. The Mexican Dollar was widely used throughout the China trade and was even legal tender in many spots in the US. Form Harris's perspective, he was pushing for Japan to be integrated into the Pacific economy, thus opening the country up.

The problem for Japan was that Harris insisted that the exchange be done by weight given that the Bakufu's hard currency was not homogeneous. This created an especially acute problem for the Japanese as the silver premium in Japanese coins was much higher than the global standard; Japanese coins used a 1:5 ratio whereas the global level was approximately 1:15. By insisting an exchange based on weight, Japanese merchants had to give up more valuable currency and received much less valuable Mexican dollars in return. The Pacific traders in the region almost immediately recognized that there was an enormous profit to be made in exchanging their Mexican Dollars or other Western coins for Japanese currency. Stopping off in the newly established treaty ports to exchange currency soon became part of the routine for some of these merchants. The weight-based exchange led to some severe discrepancies. For example, at one point the Japanese had to exchange 311 pieces of Ichibu silver for 100 foreign coins. Some clever merchants also took to taking advantage of this very unbalanced currency exchange system to use their newly acquired Japanese silver coins to buy Japanese gold ryo. Like the Bakufu's silver coins, Japanese gold currency had a higher premium on precious metals and thus more valuable on the global market. The profit margins from the Japanese gold trade ranged from 50% to 137% and caused a significant exodus of gold out of the country. Between 1860 and 1861, more than 5 million silver dollars worth of gold had been exported from the country from the Yokohama Treaty Port and replaced with a currency of questionable or lesser value.

This outflow of precious metals came at a very inopportune time for the Bakufu as it needed such capital to buy foreign weapons and technology. Edo responded to the outflow by debasing its currency, which was often termed Doro-gin (mud silver- a pun on the pronunciation of dollar and the Japanese word for mud), from melted down Mexican Dollars. But the fact that pre-Kanagawa money remained in circulation encouraged counterfeiters, further undermining the legitimacy of the Edo government. The flood of foreign silver and the export of old coins meant that bad money drove out the good and the Bakufu's debasing of the currency and use of paper money led to inflation. This in turn led to a wider economic disruption and put the already fiscally-strained lower ranking samurai class (they were paid a fixed stipend of rice which already was not keeping up with the pre-Perry economy) under greater pressure.

In the long-term view, the shocks from the Western currency exchange forced the Japanese to adapt its currency to global standards. The Sakoku system, which was really not as closed as some Western commentators would have it, really did not prepare the Japanese currency for the scale of global trade that occurred in the nineteenth century. But this currency shock and the subsequent economic dislocation it caused was one more burden the Bakufu had to bear. By prompting an exodus of gold and silver, the treaties destroyed one of the bases of the existing Japanese state: its currency. The Shogunate's negotiators did recognize some of the problems inherent in the weight exchange system, but the need to gain concessions with Harris over the land issue and the Treaty Ports took greater priority. Not surprisingly, some of the first acts of the Meiji government were attempts to put its economic house in order and it issued a new currency, the Yen, whose premium of precious metal was much more in line with global standards.

As for being "ripped off," Harris likely thought he was doing the Japanese a service by forcing them to be integrated into a global economy. His own experience having to deal with Japanese currency in Japan probably colored his own interpretation of the Japanese fiscal situation. Harris naively stated that:

no foreigner will come to Japan for the purpose of buying gold and silver, because gold and silver do not benefit a country, but a foreigner could make large profits by buying camphor, tea, lacquerware, copper, and textiles and exporting them to foreign countries.

The historical record shows that Harris wrong on this account; foreign traders would make profits off of both goods and precious metals and in many cases it was the Japanese who got the short end of the economic stick in these exchanges.

Sources

Auslin, Michael R. Negotiating with Imperialism The Unequal Treaties and the Culture of Japanese Diplomacy. Cambridge, Mass: Harvard University Press, 2006.

Frost, Peter K. The Bakumatsu Currency Crisis. Cambridge: East Asian Research Center, Harvard University: Harvard University Press, 1970.

Jansen, Marius B. The Making of Modern Japan. Cambridge, Mass: Belknap Press of Harvard University Press, 2000.