[Wauthan]

Well Firegems are more valuable then earthgems. Perhaps lowercost for earth to *1.25?
Let's try the formula on a Firesword then.

C1=5? (A fire gem sells for 15 gp but costs would be immense for units).
x=5
z1=a bit redundant perhaps?
Gold cost is then 5*(1.5*5)=37,5 or 38

c2=base item resource (broadsword)?=4
z2=not needed.
Resource is 4*(1*1.5)=6

So an ordinary Commander (30gold 14res) with a Firesword would cost 68 gold and 16 res. Unit got +6 damage +3 attack for the increased cost. Too expensive?