Depends what you call a 'return on investment'.
If you develop a very high end car and it costs 100m to develop it, then you only produce 10 of them (sold for 1m each) then superficially, it's a loss making project.
However, by having that 'supercar', the sales in your other more mundane models are boosted, to the point where it brings in 150m, then it's paid for itself.
If you're an accountant, you'd probably question the value of the 100m loss making project, but if you're a marketing person, you'll be arguing that it's a PR coup that boosts your image.
In the same way, MCFC needn't be a profit making business (although I accept that ideally the Sheik would like it to be). The bigger factor is how people are now viewing Abu Dhabi's way of working and their business acumen. Whilst your average bunch of blokes down the pub are saying 'they've done a fantastic job with the stadium and academy', there are probably some serious business partners also discussing the same in the boardroom - e.g. 'These guys seem to get things done in a very professional manner'. That's only going to result in better business relationships and ultimately profitability in other Abu Dhabi based ventures.
Of course there's a 'return' for the Sheik. He's not bought City as a pure vanity project. It's just that his 'return' doesn't have to be very simplistic one directly from the club.
I believe City need to show continued success (trophies) though. Of course we can't win everything, but we need to be winning fairly regularly in order to keep the image up. Otherwise we're a failed project (in most people's eyes). If we're in CL every year, doing pretty decently (Quarter finals+), and winning a trophy per season (average) we'll be doing fine.