Edo period Japan went through a series of monetary crises due to the lack of understanding of economic principles by the Tokugawa Shogunate, a horrible budget situation... and also a desire to centralize power away from the daimyo. Inflation was seen as a way of indirectly taxing the daimyo, which they couldn't do directly for various reasons.

A koku was nominally the amount of rice needed to feed a man for a year (330 pounds of rice), but the actual value of that fluctuated from year to year. (If there's a famine, then a year's worth of rice can buy a lot more other stuff.) A ryo (which was a gold coin) was originally worth four koku, and then debased to one koku.

So how much was it worth? Direct comparison in modern terms is impossible, but we can try.

If based on the rice standard, it is worth 330 pounds x .50 dollars per pound (wholesale is around 20c, full retail one dollar per pound) = $165.

If based on the gold content instead, it had around 9 grams of gold due to debasement. So 9 grams x $42/gram = $378

If we exchanged the ryo for 200 grams of silver or so, then it would be worth 200 grams x .66 dollars per gram = $132.

According to the Wikipedia page for it (http://en.wikipedia.org/wiki/Ry%C5%8D_(currency_unit)) in modern terms it was worth somewhere between $30 and $4000. It's a pretty wide ballpark, but that's kind of how it is.

If you're interested in the subject, The Dog Shogun goes into the reasons for the economic crisis.

During the late Edo, 1 ryo was 18 grams of gold. Japanese currency around Edo then subdivided: 1 ryo = 4 bu = 16 shu. Around the Kyoto-Osaka region silver was used rather than gold. The standard currency was monme. One monme was worth 3.76 grams. 1 monme could be divided into 10 fun or 100 rin. 60 monme would make 1 ryo.

Throughout the Edo period the exchange rate between silver and gold remained relatively stable. But not all regions used the same currency and prices could fluctuate quite a bit. Copper currency (called mon) was used throughout Japan. But it was rapidly declining in value throughout the nineteenth century. According to Teruko Craig, translator of Musui's Story, "in 1805 one could buy a decent lunch of mushrooms, pickles, rice, and soup for 100 copper mon." She lists the price of 4000 mon equaling 60 monme or 1 ryo. By the 1830s 1 ryo was worth 6,500 mon.

Craig goes on to say that by 1825 "1 koku of rice sold for about 63 silver monme. 1.8 litres of salt was 32 mon; 10 [radish] 258 mon; 10 peaches 15 mon; 10 pears 70 mon; 6 apples 32 mon; and a bunch of carrots 5 mon." An adult could use a bathhouse for 8 mon and a child could do the same for 5 mon. By 1835, a person could buy 1 1/2 yards of wool cloth for 6 1/2 ryo.

Craig says that a carpenter would earn between 420 to 450 mon a day, which she says was considered a "decent wage." However, by 1866 the copper currency had devalued enough that it took 7,000 mon to make 1 ryo. A trip to the bathhouse cost 16 mon.

I'm unsure how you could translate that into the modern day. I'm no economist and I don't really want to inflation adjust a foreign currency 150 years. However, knowing the price of everyday goods and services should give you an approximate idea of how valuable a ryo was.