Hi There
I've seen some 24" macs go upwards of £800 on ebay, but they've been some of the latest models. Since the one you're on about has a small amount of RAM and a pretty small hard drive - unless its an SSD - and I'm guessing FCP 7? (for the love of everything don't upgrade it to FCP X, I think that needs a Sandy Bridge i5 at minimum)
Would probably say around £550ish if I'm honest, but then again, I'm a cheapskate. I made a hackintosh.
Why do you ask?
Rob