From what I've heard (and VERY few semi-pros/pros talk actual money), the highest paying market out there that (occasionally) takes genre work is Playboy. I think, however, they have stopped accepting unsolicitied contributions.

I've found markets to be pretty variable in paying. Some pay on acceptance, some on publication, some... after publication. A couple haven't paid me at all, one because of contract mix-ups that were partly my fault, the other for no reason I am aware of (possibly just oversight).

A couple of times so far I've been paid over $200 for a story (my best sale is nearly $400 but hasn't paid yet as it's pay on publication and publication isn't until 2010). The least I've been paid is $3 (for Every Day Fiction). I never sub to non-paying markets and only rarely to markets offering less than 1c/word - I sub to EDF because it has a higher readership than a lot of magazines and gets me more feedback than most markets, but I'll almost always try other markets first.

I keep yearly figures. Stories that I sold in my first year netted me around $220. In the next year, it was around $580. This year, so far, I'm around $800 - but that's value of sale, not income (I am actually waiting payment on at least $700-worth of stories and may not see most of that for a good while yet). Those figures don't include the IGMS anthology, for which I was selected without even having to submit, and which has paid me an advance on royalties (my only contract of that nature to date). Whether it will earn out, and whether I'll end up seeing actual royalty checks coming through, I have no idea.

Oh, and the highest actual rate I have received is 25 cents per word. Unfortunately it was only for a 100-word story...