There is so much wrong with this post in my opinion. Firstly even if Moyes bundled through Bayern, that doesn't mean his campaign was a success. He was dogshite throughout, he didn't look at the areas that needed improvement and he single handedly managed to regress the mentality of what was recently a title winning squad. If you want to monetise how much that set us back, you'd be looking at figures far higher than Mourinho's investments this season.
A vital part of management is to look at the areas needing improvement and work on them. Moyes' inept transfer dealings is part of his own downfall (amongst many others). He gave Rooney a 5 year contract when he obviously had 1-2 seasons left in him and he went out and bought Fellaini because he couldn't tie down Herrera whilst ignoring Thiago. Money spent has little to do with it - I'd rather overpay and get the right player than under pay and get someone shite we try and find a place for over 3 season (Fellaini). This is aside from the fact that the man had zero tactical knowledge to utilise his players in the first place.
As for ROI. Its a bizzare manner of looking at ROI in terms of Mourinho's signings but lets evaluate them. Zlatan's has been great, Bailly's has been great. Mkhitaryan and Pogba's have been so so, but they are here for multiple seasons. They were better than the previous turd in that position (Depay, Schneiderlin et al.) so I'm not sure why you're complaining of ROI.
You also can't limit the ROI for just one season and judge signings when you know there will be higher expected returns as seasons progress. Mourinho had money to spend and so did Moyes. Moyes never spent it and regressed the mentality of our squad, brought us well outside of top 4 by March and left an absolute mess for the next manager.
I never said Moyes would have been a success if he beat Bayern, but that would have been an achievements people would have remembered him for like how LvG's remembered mostly (Footballing wise, not the drunk rant at the end of season awards) his displays against Liverpool, Spurs and City.
If you monetize how much Moyes spent, I agree it's a lot, but it's still nowhere close to the expenses Mourinho has incurred. Moyes was manager at a time when there were lots of players on huge paychecks. Vidic, Ferdinand, Rooney (Yes, Stupid decision btw), Nani, RvP, being just a few examples.
I know everyone hates Fellaini and some people are always too stubborn (Not suggesting you are), but he's being relied on heavily by Mourinho. That's two managers now (LvG, Mou) that have felt he has an important role to contribute in the short term goals of the team. And even though, you called Moyes's transfer dealings as inept, he
did sign two players who were being trusted by the previous manager and the current one (Mata maybe not so much). So whilst he didn't get the players he wanted, the one's he signed haven't been too bad. Mata from what I read, had a better Goal Scoring ratio at United than at Chelsea. Agree that Mou's transfer dealings are yet to be fully evaluated, but as it stands, he's done average or above average so far. No doubt, it will improve for the likes of Mkhi and Pogba.
ROI individually, yeah, Zlatan and Bailly have been smart deals. Even Pogba has been a good buy (We look dire without him). Like I said individually, they Mou has bought well. But as a team, we have been underwhelming. It's one of the jobs of the manager to bed all the new signings with existing one's and in that respect, I think Mou has failed. That's why I rated his transfer business as ok.