I road whored pretty heavily for a period of a year or two.
Best advice I can give: Look for new races.
Even a five year-old 5k putting up $250 for first will usually draw something under 16 minutes. And maybe under 15.
It also depends on where you are. I did most of my running in New England and regularly had to contend with B-Grade Kenyans, Providence grads, and the pretty hard core track clubs. If I wanted $500, I had to go 15ish or faster, 31ish or faster and 52-53 minutes for 10 miles or faster.
But definitely stay away from the big, established races. These almost always put out decent cash, and they *will* attract talent. Also, look for $$$ races within a week - ideally a few days - of other big events. That way you clear out a lot of the guys who would normally come for the cash, because they've already raced. Or, at the very least, they do end up racing you, but they race less than 100 percent.
I won money in three separate 5ks over the course of four days and didn't have to break 16 minutes once. I did, however, do a decent amount of traveling.
Good luck.