I think I found places to stay in Frankfurt this Sept. There's a huge auto trade fair going on when I arrive and all hotels are booked solid. The whole point to land in Frankfurt is to spend a bit of time and have dinner with someone I knew from HS. Anyway, another old friend from summer camp threw up a link on FB to this site that coordinates renting out rooms/apts/houses, whatever. You basically rent the spare room of someone's place (or the entire place if they want). It's cheap! I put in a message to a guy that lives in DT Frankfurt. Hopefully he's spare room is available. If not, there's a number of other people there renting out rooms.
http://www.airbnb.com/