Economy of scale curve

I am developing tools to help do preliminary estimates of remodeling projects. By it's nature, a smalle project will cost more / square foot than a larger one. Lets take a deck as an example: I have a 100 sf deck that I've completed that cost $40/sf, a 200 sf deck that cost me $20/sf and a 400sf deck the cost me $15/sf, (A,B,C). (We can assume that the smallest deck I would ever build would be 50sf and the largest would be 2000sf). These knowns, will create a curve on a graph. The curve will generally look like the attached spreadsheet. It will never touch the x or y axis. The A, B and C points will all be related so the curve shouls have a shape similar to the image.

Assuming that I know A,B &C and I have a minimum and maximum size, what formula would I need to calculate what a 325sf deck would cost, or any other size deck between the smallest and largest parameters?

Thanks, This Helps!