Not necessarily. A lot more silver has been mined since the Norman Conquest (for example in the New World, but also in the Old World) which increases its supply. The demand for and utility of silver in general has also changed since then.
Picking the 'BigMac index' might be fun, ie you try to price a Big Mac. (Since the Big Mac doesn't contain any tomatoes, you could probably have made a reasonable Big Mac clone in the Middle Ages.)
It gets ridiculous, if you go by the price of eg the amount of computation a human can do in a year. Or 'ice cubes in the height of summer'.
Yes, but they were available in the Middle Ages.
> And there are tomatoes in the special sauce.
OK, I didn't know that. They didn't have tomatoes in the European Middle Ages before contact with the New World was established.
But you can probably come reasonably close enough to a Big Mac knockoff with stuff they had available in the Middle Ages.
Edit: per an up level comment, 15 chickens or half a knife? Hmm, chickens are easy the knife isn't. (wide variability in knife pricing). Given that the 'collectible' value has increased beyond the monetary value.