What level quests are you doing? I've found medium quests to be extremely profitable, once you get past the initial level build-up. Hard quests are less so, but still turn a PG profit without having to sell the BP. On the other hand, I've found that I have to sell BP and items to break even with impossible quests, and I'm not sure there's any way to make them profitable.

That's with running my own contests, though. If you take the time to enter public contests and choose splits that your ponies are near the top of, they should level even faster than they would have with private contests.