Most of the ones I've seen sell in running condition went well over $1500. The ones with blown engines (but otherwise in good shape) are usually advertised for $500 or more. The ones in poor "parts unit" shape usually run at least $300.
As mentioned above the cost of overhauling the Onan engine is very high (nearing or exceding the cost of a cheap Chevy small block overhaul). Unless you have have the PTO or the hydraulics I'd rather have a 1650 for ease of ownership (and you can add the hydraulics to the 1650).