Credits isn't the same thing as either revenue or costs. Look at kids pirating Photoshop and Maya to learn the tools and make cool shit - that's not really lost revenue, because kids just don't have that kind of money to spend on professional tools to begin with. Most open source projects taking Vercel credits don't have the money to pay Vercel in the absence of credits, so this doesn't result in lost revenue. And it's not the same as costs, because if it did, then Vercel wouldn't be making a profit.
The only reasonable way to track this is to understand how much each customer costs (in servers, hardware, etc) and internally attribute those costs to marketing instead of revenue.