It's hard to judge without pics...You don't see a whole lot of 455 Formulas. I'd venture to guess, based on your description, that it should go for between $5,000 and $8,000 for a decent driver with not too much rust but one that's not a perfect survivor. Maybe more for a really desirable color combo, wheels, etc.
What color is it, by the way?
Got any pics?
EDIT- Check this one out. Apparently sold for $10k
http://www.cars-on-line.com/17767.html