Seriously, its fricking annoying to see people whine to developers about that. Why the heck people think that 3d is the more expensive one? If 3d was more expensive than sprite games, then sprite games would be more common.
Edit: Sorry, made this thread when I was in crappy mood, I really shouldn't talk about things when I'm angry.
I think you're confusing 'cost to make' with 'performance' on old systems (like mine)
I can easily run games like The Witcher Enhance but my old PC chokes on recent SNES and NES style games like Gorril Slayer and Offspring Fling.
It's a pretty strange phenomena and I'm not sure what causes it but CPUs used to have a more difficult time running 2D games at full frame rates along with full resolution.
And old debate used to be about how the Sega Saturn was more competent at running 2D fighters in full res where as the PSOne had to drop the resolution down to keep up with Saturns frame rates.
As for expense of creating sprite games though, look up these programs: http://www.yoyogames.com/make http://www.gimp.org
Total software investment is $50 for liscence to publish with Game Maker
And once you learn how to use these programs you can pretty much spam 2D games like these kids did: http://sandbox.yoyogames.com