I think there is too much competition and you will end up spending too much time on it to see a beneficial return. But you never know, if you provide a better service than the comp then they could stick with you.
Just thinking. How about to spend some time on flippa.com and catch couple good deals and try to grow already pre-made sites, investing more money in it? Few good sites could make some profit all together. And I'm not talking just about Adsence.
Sorry for offtop, but why people selling some sites, which could bring, for example, a $300/month for like $600-700? So cheap. Or those sites does not making that much for real?
Use your common sense when you are buying a site. Beside that look for facts, track back history and do research on the internet also check out the seller his history on the website where the auction is.