They usually pay the standard rate that would be applied for the expected amount of hours of completion of that task, with a small plus. this way, even if you don't land the job you still don't get out of it feeling like you lost invaluable time of your life.

But in my own experience I have once worked for free on those tasks, hoping to get hired, and when it didn't pan out it made me a bit sour. Won't do that again.