A small number of boutique advertising networks have successfully tackled this challenge and are making quite a lot for their trouble.

For instance, The Deck (http://decknetwork.net/): 30 ad spots per month, currently $7,900/ad, equals $237,000/mo (and they're always booked solid.) This doesn't even count their "roadblock" ads which are an additional $7,900/day. The best part about their ads are they're good. I've found quite a few excellent products through them over the years.