Well, one option is time-boxing. You say, "We'll put a team on this for up to two months: they get done what they get done." It's a fixed investment; it's up to the business stakeholders to judge what kind of return they can get out of that.

But in the this context, by "estimation" I mean "formal estimation". Having developers glance at something and ballpark it is basically a zero-cost activity. As long as that I number has about the same error bars as your R number, then it's perfectly fine for ROI. Especially given typical requirements volatility, and a project approach that sharpens ROI estimates as you go.

You don't have to know the ROI value of each project, just compare their relative sizes to select the best from the set of projects with the highest profit ranking.

This assumes, though, that the best of the projects is worth building in the first place.