The 29" Neo and Super Neos seem to go for ~$400-600 without the Neo-Geo motherboard but otherwise working (monitor, controls, power supply, coin mech, etc.) and in decent but not factory new condiiton. A 4-slot motherboard in tested working condition seems to vary between $75-200.
I've got two Neo 29s. Gave about $500-600 valuation (cash + trade) for each exclusive of shipping.