Ultimately I won't talk about what he's worth, because that's up to whoever has the money for him, but there are quite a few players in world football that are better, younger, and cheaper, and if you really have that kind of money why wouldn't you just pay the asking price for Koulibaly?
While I agree with you, I think it's worth adding another quality where Maguire suffers by comparison with others - European experience. At 26, he's played fewer European games than John Stones had when he signed for City as a 22 year old.
I'd forgotten Maguire was 26. That's hardly ancient for a CB, but it's a reminder that his election to the PFA's League One team of the year in 2011-12 as a nineteen year old didn't bring the rapid rise that was mooted for him at the time. Given Maguire remained a regular starter at Sheffield United, I often wondered what the knock on him was, before I duly read it was a lack of pace.
And there's another tricky matter. Van Dijk is an athlete, and that equipped him to protect the vulnerability inherent in Klopp's style of play - the foot races that occur if the opposition springs the press. Even if we accept that Maguire is the best CB outside the top six, that means he's the best of a bunch of players who mostly play for teams that don't leave a lot of grass behind their back four, and who do their best to have a block of bodies ahead of their defenders too. Leicester is a case in point - a counter-attacking unit that affords their CBs a good deal of systematic protection (a necessity when so often you field Wes Morgan). Assuming Solksjaer wants the Fallen Empire to be an attack-minded outfit, Maguire may find himself more vulnerable defending greater areas of vacant space in front of him than ever before in his top flight career. He reads the game pretty well, but Bobby Moore he is not. Nor is his skill on the ball so high that he seems like a ball-playing CB whose rewards justify the defensive risks.
Funnily enough, I'm old enough to remember Manchester United paying a ludicrous sum for a young English centre-back and coming up trumps. That was Gary Pallister, in 1989, when £2.3 million was the second biggest transfer fee ever paid by an English club. But the differences between Pallister and Maguire outnumber the similarities. Those were the old, Football League days: the competition, Liverpool apart, wasn't so strong either financially or from a sporting perspective, and, anyway, the bridge between top and bottom of the league was far smaller. Pallister was 24. Most importantly, he came from a Middlesbrough team that under Bruce Rioch played a passing game, and he possessed far greater mobility and a better turn of pace than Maguire can call upon.
Ultimately, 'worth' translates to whether a player's presence significantly improves a team. It's hard to see Maguire fulfilling that function at Old Trafford.