I agree. Ball Pythons spend their time in termite burrows, and other pre dug holes, they are also nocturnal. They aren't out basking in the sun.
Even snakes that are out and about in the day sometimes, such as a cornsnake maybe, use the heat from the sun as well as heat coming off rotting vegetation piles, hot roads, rocks that hold heat well into the night, etc. Effectively meaning the take above and below heat.
In the end personally, I think heat is heat and you can be successful with either. The problem with overhead heat for ball pythons specifically is that it has a serious drying effect. As well as being extremely bright for a hole loving, underground living snake. I would use a light for a king or corn before I'd ever use one for a Ball Python.
As for costs, I assure you my 30 watt heat tape on a thermastat takes up FAR less than my 100 watt bulb for my leopard gecko.