[The_Tauren13]

OK, scratch that old and busted formula; that was just something basic to start off on. Here's the new hotness:
sqrt(((HP*(10.0+Prot)*Def*MR)/1000.0)*((Str*Att)/10.0))*((3.0+Move)/5.0)*((40.0+AP)/50.0)*(9.0/(6.0+Enc))*((Mor^log10(7))/7.0)

Comments:
HP, Prot, Def, MR, Str, and Att are all wieghted the same now.
The square root does not encompass Move, AP, Enc, or Mor.
log10(7) = 0.84509804001425683071221625859264 (about), meaning that about 22 Mor is worth twice as much as 10 Mor (instead of just 20 Mor being worth twice 10). I chose to use a logarithm with base 10 so that, for unit with 10 Mor, the equation (Mor^log10(7))/7 will equal 1, so a unit with 10 Mor will be 'average'.
The basic idea is: sqrt(defensiveness*offensiveness)*other. I decided upon this because in a simplistic system where the units only have 2 stats, HP and Damage, an ideal cost formula is sqrt(HP*Damage).
I counted MR as a defensive capability, but it could be brought out of the square root to be weighted more heavily.
I didn't really know what to do with Enc, so I just threw it in so that a unit with 0 Enc is worth 50% more than the same unit but with 3 ('normal') Enc, and the same unit but with 12 Enc is worth half as much.