Have you looked at any new home builder sites by you? That might at least give you an idea of how they do it.

Your formula is fine assuming you are putting in lot developement costs as cost of land and adding in for upgrades above builders level.

You'll get those numbers from your local developers. Your Ins. agent might help but I'd take it from developers or a realtor you can trust. An appraiser if you can get one to tell you. Barring all that, cruising some displays with a careful eye and a note pad. Options usually have high margins ... be careful of too much extrapolation.

And after you're done with all that, you can throw half of it away :-)

Kidding ... That just tells you how a place is priced relative to new const. Then there are the near intangibles, depreciating everything for time and condition, then adding back in the lessor risk of existing over new and local market conditions short term and long.