Ummm. We're talking transfer fee plus potential bonuses to City for performance. Which was max 12 mil if I recall correctly. When comparing transfer fees you don't include wage payments and other such things, just the cost of the transfer. It's amazing that Higuain cost Napoli 37 million, transfer fee only. Tevez might cost us 12 mil. Higuain also makes higher wages. So according to your logic, Higuain might actually cost Napol about 80 mil over 5 years.
Because the actual transfer fee for Tevez (including bonuses) was so low, he came exceptionally cheap compared to players of similar goal-scoring prowess, that would have commanded 30 million plus transfer fees plus similar, if not higher, wages.