How Complicated is it to Add a Square-Footage Calculator to a Site?

[SteveO]

I have a client who needs a web site. I'm the writer on the job. He told me he wants a square-footage calculator to figure area (he is a building materials supplier) so customers can get an idea of how much their job is going to cost. He said that a site designer told him this is very expensive to put together. I can see how it would be if you did the coding from scratch. But it would seem logical that a "stock" calculator from a web design tools site (I assume these exist) could do the same thing for a nominal fee, if not for free.

Any web design folks care to comment? Suggestions on where this might be found? TIA.