The excellent blog Pimp My Novel
recently wrote up a State of the Union of Crime Fiction of sorts. Click here to read it
. Here are some of the highlights as copy/pasted by me:
1) Mystery sales account for almost a quarter of all adult sales (units), so as long as your writing is solid and you've got a more-or-less original idea, you should be okay.
2) From what I hear, [crime fiction is] a great place to be over the next few years, so if you're writing in this genre: carry on.
3) From what I've observed in national account adult sales, the average advance for mystery/thriller/suspense is roughly in line with the average adult novel advance, which is to say, around $10,000 or so (in my experience, closer to $8,000). You could make as little as $1,000 on your debut novel and as much as a couple million; it all depends.
4) If your mystery/suspense/thriller is coming out this September (particularly in the second half of the month), you might be a little overshadowed. Otherwise, though, you should be fine.