What follows is an attempt to give some sense to value, the dominant
forms of value exchange and the types of currency in which wealth was 
extracted. There is wide disagreement between my numerous sources, so 
this is only a very rough guide that I wouldn't encourage being taken 
as historical truth (though in many instances I think we are probably 
close to the reality) but should serve for game purposes.

ell = standard measurement of cloth or wool somewhere around 2-3 foot
long*
a "hundred" = actually 120 ells of homespun wool
ounce = 1/16 pound
pound (weight) = 2.2 per kilo
mark (of either silver or gold) = usually 8 ounces
pound (money, English) = 40 shillings = 3 marks
12 pennies (English) = 1 shilling**
1 ounce of gold = 8 ounces of silver
120 ells = 8 sheep (or 1 and 1/4 milking cows)
milch (milking) cow = 6 sheep
1 sheep = 10 pennies
1 cow = 5 shillings = 60 pennies
120 ells = 80 pennies 

* (though I believe the measurement is derived from the swedish word
for the distance between wrist and inside elbow, much as foot is 
derived from heel to middle toe).
** some sources believe it was more like 5-8 pennies per shilling,
the dozen coming along only at a much later time in history