Me and my friends had 4 nights at Antonella's cozy apartment. It was clean and easy to reach, 3 minutes from the station Campo di Marte, and 8 minutes from the Bus stop.
I recommend Antonella's apartment, it's cheap, clean and big enough for 4 people.
The only thing that you may get annoyed from is the noice which comes from trains, there is only 1 street between railway and the building. But since it's Florence, and most of the time you will spend in city center it doesn't matter at all. :)