There were two such guidelines I can remember. One was Illwinter's, and other came from analyzing mages. Both are out-dated, as commander price guidelines don't work with Dom3 economy.
Code:
Soldier cost
Type Att/def Mrl Gold
Militia 8 8 7
Regular 10 10 10
Elites 11 11-13 13-15
Special Elites 11-13 12-15 15-25
Sacred units generally costs 50% more.