I don't think stock is any cheaper than mod. Stock motors are not THAT much less expensive. And, for serious stock racers, they need BETTER batteries than in mod (except run time) because the voltage makes such a difference. Also, in stock you got guys running plastic out drives, aluminum, and other lightening hop ups that only cost more money.
I personally like to run stock, but I also run alot of mod. I enjoy the "level playing field".