I would check out prefab/manufactured home.
Cost about $50sq/ft top + $30-40sq/ft(prep sitebuild/foundation)= ~$100sq/ft.(land not included)
so if ea unit is 1000sqft x 4plex = 4000sqft x ~$100sq/ft = $400k
Contractors cost for presite build work varies: eg. concrete foundation.
A reasonable quote would be around 30-40% of the prefab building cost = ~40% x $50sq/ft = $20sq/ft
Guess anything is possible --- but where are you building, what is the cost of the land, what are you building, what finishes are you selecting, etc. Are you prepared to devote time managing the construction project and be the general contractor?
While you're mulling over your decision, perhaps you should visit Remodeling Online'st Cost vs Value --- cost of remodeling and how much you can expect to recoup.