Perhaps we're talking at cross purposes here; when talking about ROI, the numerator and denominator are usually both in dollars.

If I'm pitching Lilium to some VCs, I'd model out the cost of researching, developing, and building the fleet ("I"), and the revenue I hope to generate in a given timeframe ("R"). After adjusting for the risk of failure, if that ROI is higher than what you can get with other investments, the ROI is, in the relative sense "good". If it's worse than putting your dollars into treasury bonds, it's "bad".

Paraphrasing, your arguments earlier in this thread were that investing in aviation is brave, because the ROI on those investments is bad, because it's expensive to certify new inventions. I certainly agree that it's expensive to build a plane, or even add features to a plane, and even more expensive to build and certify a new plane. But that doesn't mean it's bad ROI. The return generated by building expensive things can be astronomical; see military and aerospace contractors taking in multi-billion dollar contracts with very healthy profits, for example. There should be clear numbers behind this; how many aviation startups succeeded in the last N years? How many folded? That would be the objective analysis I'm looking for when talking about ROI on investments in aviation companies.

Maybe that's not the point you were intending to make; perhaps you're just talking about the ROI for an individual buying a plane, rather than a business operating a fleet (like Lilium in the OP)? But if that's what you mean by ROI, then I don't think that is particularly relevant to investors; they are interested in ROI in the sense I outlined above.