depends on your stats partly. I'd say 15 is roughly average, and 20 definitely isn't out of the question, especially if either you have a hole in your app (GPA or MCAT not so hot, not much research or clinical, weak ECs, something like that) or you live in an area where the competition for schools is insane (i.e. a lot of highly qualified cali kids are applying to 20 plus schools as far as i see it).
I would err on the side of caution. If you skimp on the apps you may end up having to apply again the next year. Why not overcompensate a little? Sure money is an issue, but spending a few hundred (or maybe a thousand or two) more dollars will really be pretty small compared to the amount you will owe for four years of med school. If you think about applications costs and the actual cost of your med school as one number, what's really the difference between $100,000 and $101,500, for example? I say just go for as many as you think you can handle. Now some people have said before that you don't want to overextend yourself and have so many applications that every individual application suffers. I think if you really care about certain schools then you will take the time to make them your best. Maybe even pick a portion of your schools to be "work-your-ass-off" applications. Or maybe just do that for all of them, which I think is possible if you plan your time well and get the ball rolling early. Oh yeah, and I really think after a certain number of applications you will pretty much cover all the possible questions, or will in general. So maybe those last 5 or 10 apps will be cut and paste with some easy modifications. All I can say is that right now I think in a year I will be applying to maybe 20 to 30 schools because I think it's just the safest bet.