At one time I got annoyed by this process because some of the "sites" I built for a test lead to building the clients website for real from the test (which I built during the test) -- but the site would be totally different because at least you'd get a chance to talk to the client -- I've even come close to the point saying in interviews "Sure I can do this, but it will cost you £x".
I never did though. Probably didn't have the balls to say I don't do work on spec... I mean would you go to a company's test and say sure I'll do your exam, but you're going to have to pay me for my time?
Sounds silly, just shows you how annoyed I got at the whole interview process and just started to work for myself instead.
But I'm sure you'll get the job...
Good luck in your interview anyway.