I've heard Castles do this sort of comparision of how much Utd have spent on their defenders, compared to City but like him you're ignoring the fact that Utd spent considerably more on other positions than City and City are stronger there too. You have to look at what's been spent as a whole - Utd/Mourinho chose to sign Bailly and Lindelof (he actually cost €45m, Benfica released the exact details of the fee) instead of more costly defenders and instead spent their money on far costlier midfielders and attackers. For example, City signed De Bruyne and Stones for less than Utd paid for Pogba and Bailly. City signed Laporte and Jesus for less than Utd paid for Lindelof and Lukaku.
Maybe had Utd not signed Pogba and Lukaku and instead bought players that are out performing both and cost considerably less then they too could have spent the difference on their defence.
Thats completely ignoring what United had vs City when Pep and Mourinho showed up.
City had and still use:
Aguero
De Bruyne
Silva
Kompany
Fernandino
Sterling
Otamendi
Thats a selection of top quality players that still play a huge role in the Man City first XI.
Compare that to United:
DDG - top quality, has been a huge part of everything we have managed to do in the past 8 or so years
Rashford - jury is out. Can be great or completely useless
Martial - masses of potential but seems to have an attitude problem
Smalling - makes mistakes far too often and not great even ignoring the mistakes
Jones - see above and add regular injuries
Lingard - shouldn't be near the first XI but shows our lack of options in that position
Mata - has never looked good enough at United, slow, looses possession too easily. Very poor mans Silva
Valencia - alright right back, awful wing back, shouldn't be near the first XI but no good options
Shaw - injured and just getting back to what he was pre-injury
Herrera - would die for the club but a fairly limited footballer
Young - done a job but shouldn't be any more than backup
Rojo - rash and error prone, not good enough
Fellaini - A good plan B but another example of a player that shouldn't be at United and shows a complete lack of focus when it comes to building a team.
So you have City with a spine of world class players including one of the best strikers in the world along with 2 of the best midfielders in the world and then you have United with a single world class player in DDG.
Honestly Aguero, De Bruyne, Silva and Sterling (in form) would get into most sides in the world. DDG would be first choice almost anywhere but other than that we have a bunch of backup players. I would happily get rid of everyone on that United list bar DDG, Shaw and Martial. Some of them are squad players but all of them are very much in the mix for first XI places which is ridiculous.