If it weren't for the insanely great prices Apple has been applying to refurbished eMacs at the online Apple Store, your machine would probably be worth closer to $500-550.
But with 1GHz/Combo/Radeon 7500 Refurbished eMacs for $649 and 700MHz/CD/GeForce eMacs for $499 this morning, I would say that you could possibly fetch $350-400 for your iMac. Possibly more if you use eBay, maybe.