Establishing a price for a rebuilt engine that's never been run is difficult. For the buyer, it's a crap-shoot. He has no idea what he's actually getting. Unless you have a detailed build/blueprinting sheet listing every part that went into the engine, receipts for everything, including machine work and assembly by a known and reputable shop, and dyno sheets demonstrating that it's a decent build, you're not likely to get much.
Check craigslist.org for how comparable engines are priced. I've seen 'brand new performance rebuilds' priced from $500-$1500 (certainly a fraction of what it should cost to build such an engine). I imagine most buyers of such an engine will likely disassemble it to check everything (parts, clearances/tolerances, etc.) before investing the time and effort to install it into a vehicle, so they may have the added expense of a gasket set too.