Don't figure it on a square foot basis. How many heaters X how much the heater cost, How much wire- cost, how long it takes to hook up a heater including time to get it, unpack it, clean up after it X your labor and burden rate, how long it takes to run the wire including all that goes with it, Some type of control for the heaters and associated costs. Multiply by overhead, multiply by profit if you'd like some. Might want to look into how to estimate a commercial job before you guess at a cost for 1. Can use 2 hours a heater, 1 hour a therm, 20 hrs. a M for 12-2, additional $10 each heater and therm for associated hardware. X 10% overhead X 20% profit as a basic "for other than to a general" guide. Make sure your wiring method will be acceptable to the AHJ for a place of assembly.