Whoops... I've decided I need a little correction to the medieval coinage to Prime currency chart found in the WorldBook.
Medieval England Prime
Farthing 2.5 coppers
Pence (d) 10 copper
Shilling (s) 10 silver
Crown 50 silver
Mark 132.5 silver
Pound Sterling (£) 20 gold
This is much more reasonable. I think. At least it matches up better with the famous Medieval Price List by Kenneth Hodges (here's one link to it: Medieval Price List).
This explains where they got the price of a cow at 10 gp. But if they used this list for the cow, then why not use this list for the price of beef (1 sp a lb.)? It doesn't matter - the cow just has to sell for more to make sense. Also, .75 d (or 7.5 coppers) for a lb. of wheat is unimaginable, when a pound of beef allegedly sells for 1 d. I'll have to ignore the wheat price in favor of D&D's 1 cp per pound.
Interestingly, these numbers are 10x what I originally used. Just after I'd talked about how much less valuable precious metals were on Prime.