40 grand is nothing compared to the overall revenues to be had.
It won't cost them $20 per person to make it because then there would be NO PROFIT. It'll likely cost them $5 per download, including all previous costs, to make a $15 profit per download. So they're not handing out $40,000 if there are 2,000 testers.
Secondly, it means they don't employ an official testing team which would be more expensive, so they still make a killing. They wouldn't do it if there wasn't some sort of incentive other than "being nice".