I think it's great that people are trying this. But when I'm looking for jobs, this wouldn't influence me much. It might be a useful signal that they respect my time, but also could indicate a relatively mercenary culture.

From the hiring side, I'd also have concerns. At a previous company we paid for a lot of user testing subjects and there were some people who were trying to make a living out of it. They'd say absolutely anything up front to try to get in and get the money. And that was just for $30-$50. If we're talking $200-800, that's a lot of incentive for goofs to lie their way into an interview.