I guess I'll help:
The costs in this case would be the time and cost of programming and bundling the ad, as well as the ad spend of getting the ad shown. The earnings would be in cryptocurrency. Any non-zero amount of cryptocurrency earned would mean you have a return .... on investment, and any cryptocurrency amount now or in the future that is valued at more than your costs means you have an ROI greater than zero.
Mining pools count shares of work toward the pool itself finding a block, based on processing power contributed to a pool. So it doesn't matter if users uncovered and uninstalled your mining-ware after 5 minutes in the most technical savvy outlier case, the shares of their processing power was already submitted to the pool and that account got paid.
So wouldn't it be hard or almost impossible to say they have an ROI of zero? They are probably making a good amount of money.