Using a bit of googling it looks like a £ in the 1300's was, as the name states, a pound of silver. The exact amount of silver varied a bit over time but this comes out to be about 350g of silver or ~$200.
So a War Horse at £80 is $16,000 Axe: 5d = 5(200/240) = $4 Cow: 10s = 10/20200 = $100 Dozen eggs = 1/2*(200/240) = $0.4
Not too far off. Could be just a historical accident, but with the value of silver holding up over the centuries, I can see where the goldbugs might get their obsession with metals.