I was wondering if anyone knew of any good resources that would tell you what percentage of gamers like fighting games, strategy, turn-based strategy, flight sims, RPGs, casual, etc..

This would be useful information to know for calculating market potential, but I'm having trouble finding anything that isn't expensive: Most of what I've found requires payment.

Yes, good sources of market research require money.

There are sites like vgchartz.com that can help with some basic research, but more refined searches take real research which takes an investment of time or money.

It's also worth being very wary of any survey that claims "X% of gamers like Y". Self-reported results are often biased and claimed preferences may not line up with actual spending. Additionally, the surveys may not use rigorous or even useful definitions, and the purpose of many surveys is for social science or political ends rather than market research. So, be aware.

Hermm.. therefore when calculating market potential, you're just doing a bunch of wild-ass guessing.

How 'wild' it is depends on what exactly you're doing, and with what data. Buying decisions are complex and looking at the size of the potential market is mostly just about getting a ballpark figure. It's useful for publishers and investors who can use it to guide their long term decisions. It's not so useful for gamedevs who want to guess how their own game will perform, because so much is down to individual factors - factors that may average out over the industry or genre as a whole, but could differ wildly from game to game.