To tell you the truth, an inexpensive Mac that'll run FCP, will only be found on Ebay, or maybe from a dealer. But as the above poster said, it may be in need of some tweaking.
I couldn't afford my own Mac, but my Junior High School was generous enough to give me a Power Mac G5. They gave it to me because I made a feature length video for their Athletic Association. There was no way I could have afforded it.
I'm not really sure what I'm suggesting here, but I think you get the point of it.
Yeah it stinks being a low-budget filmmaker, but don't worry, youu'll find something. Maybe if you have a relative that owns a powerful enough Mac, maybe they can let you use it some time.
Hang in there.