Estimating cost of interior apartment build out

[Timothy C.]

Working the numbers on a potential deal. If I can rent both levels it works, but only the upper level has a rent-ready apartment.

The bottom is 95% open space (think large chapel).  If I want to convert this sizeable space into 1 or 2 apartments, is there a tool or rule-of-thumb to use to estimate the costs of this project? Maybe based on square footage?

I suppose getting a contractor on-site is the most responsible route for due dilligence, just don't want to waste time for everyone involved if this is fantasy land.