As a Roblox developer, there’s currently no way to accurately calculate a return on investment after sponsoring an experience. The only relevant metrics shown on the ads manager are Impressions, Plays, and CPM / CPP, which aren’t sufficient for determining how much value was actually received from a sponsor.
Additional reports provided after a sponsored campaign ends, which include metrics such as the lifetime value and retention rates of the users who joined your experience through the sponsor, would significantly improve the usefulness of sponsored ads and allow developers to accurately determine the ROI for their campaign.