Where can I find out what things actually cost in the Middle Ages, using medieval money?

The D&D game uses gold, silver and copper coins which all have real-life counterparts, but the prices for things sure don't seem to match from what little I have discovered concerning actual medieval prices for goods. Is there a site or book somewhere that lists items with their real-life prices rather than the inaccurate D&D ones? Is there a way to convert D&D prices to actual prices? Obviously this would only cover normal items like cows, lanterns and longswords, not magic items, but I would like to know if at all possible.