The Middle Ages cover a very long period of time. The value of many things (including gold and silver) was greatly different at different periods and in different parts of the world.

Also, it would have been impossible to buy a frigate for any amount of gold or silver during the Middle Ages as frigates did not exist then. The term ‘frigate’ was first used in the 17th century to describe warships built for speed and maneuverability.

After the collapse of the Western Roman Empire, gold and silver became very scarce in western Europe.

The last place west of the Byzantine Empire to mint gold coins was Lombard Italy, and they stopped in the 8th century

This led (gradually) to the growth of a non-monetary economy in parts of Europe where taxes were paid in service or in crops, rather than in cash (the ‘feudal system’).

Some European rulers managed to continue minting silver coins (Charlemagne, for example, but others as well), but there were very few sources of silver in western Europe. The Roman silver mines in Spain were in the hands of the Arabs, and other Roman silver mines in Europe had been exhausted. So where did Europe get any silver in this period?

It seems to have mostly come from the Middle East, via Russia, in return for slaves and furs.

http://mygeologypage.ucdavis.edu/cowen/~gel115/115ch7.html

In 938, a giant vein of silver was discovered in Rammelsburg, in the Harz mountains in Germany. This was mined by Otto, Duke of Saxony, and the wealth helped him become Emperor.

More silver mines were discovered in the German mountains, and this helped Europe begin to return to a cash economy in the 12th and 13th century. Large issues of new silver currencies were common across Europe in the 13th century. Also, Italian cities began to issue the first new gold coins in Europe in many years. Partly fueled by a gold discovery in Kremnitz in Hungary in 1320.

Unfortunately for Europe, in the 14th century, some of those silver and gold mines were producing less, and the Mamluks had established a near monopoly of the spice trade, draining gold and silver from Europe. By 1450, almost all the mints in north west Europe had closed, due to lack of silver.

Luckily, the Saxons invented new methods to pump the water out of their flooded silver mines, and European silver production increased again. Then one of the greatest silver lodes of all time was discovered at Joachimsthal in Bohemia in 1516, and new supplies of gold and silver began to arrive from the new world.

Also, the Portuguese exploration into the Indian Ocean, and victory in the naval battle of Diu against the Ottomans, Mamluks, and other allies, allowed the Portuguese to break the Muslim monopoly on the spice trade and slow the drain of gold and silver towards the east (though it remained a net outflow for centuries, just not as fast).

In short, in Europe, the availability of gold and silver in the Middle Ages fluctuated a lot. So the price of gold and silver fluctuated a lot (or, to look at it from the flip side, the price of goods and services in gold or silver fluctuated a lot).

Indeed, for a long period in the earlier Middle Ages, gold and silver was so scarce in Europe that the money economy mostly broke down and was replaced by an economy of barter and taxes in goods and services.

You would need to be a lot more specific about dates and places to have any hope of getting an answer on the value of gold or silver in Medieval times (and even then, data is scarce, and valuation in long ago times is a dubious estimation at best).