I've found a lot about what kinds of taxes existed in the Middle Ages, but couldn't find a source pointing to specific percentages (apart from the tithe, but I don't know how thoroughly it was implemented). I'd like any information, however local or limited in time or scope.
In medieval times (and even in modern times) taxes were highly irregular and varied from place to place. In many instances a "tax" was really just a robbery. For example, in England since Domesday the most omnipresent "tax" was what was called the "hearth tax". The way this originally worked is that Norman soldiers went from village to village and visited each house or cottage that had a chimney (and thus a hearth). They would burst in and loot the house of any obvious valuables. This was the "tax".
By the time of Edward III who reigned 1327 to 1377 (50 years, a very long reign) the hearth tax was standardized at 2 shillings per hearth. This would be one tenth of a troy pound. Computing in gold at 16:1 and adjusting for sterling, this would be 0.0675 ounces of gold or $84 dollars per hearth per year.
However, a tax like this was just the beginning. In practice there were literally hundreds of taxes on anything the rulers could conceive of. Most of these taxes were against nobles (scutage) or against merchants (murage, pavage, pontage, stallage). Also, tolls and tariffs were common. For example, it would cost money to do any kind of transport; cross a river, cross a bridge, enter a town, land a ship. Adding up these taxes in any kind of consistent way would be difficult.