The 9.6 battery has two more cells, so there's justification for a 1/3 higher cost. The rest of the difference likely has to do mostly with economies of scale. 7.2v has been a 'standard' in RC for a long while.
Actually... your 7.2v batteries at $17.50 are $2.92 per cell. The 9.6v at $25 are $3.13 per cell. That seems pretty reasonable to me.
- Jason