Because I doubt PSG or Juve offered him lower wages than what he earns right now to lure him away from Barcelona, and 10M are nothing in the current market, he shouldn't be cheaper than Muriel, Luuk De Jong or Zaza.
The fact that you (rightly so) don't want to repeat the Matic deal with Rakitic doesn't mean his value is lower because you got burnt in a similar operation 2 years ago.
Leaving aside his fee, you don't lure away a player from a top team fighting for a treble, to a team that will fight to enter European competitions by lowering his wages.
Maybe if you're Everton or Leicester, but everyone knows United has as much money as Barcelona, independently of how much he'll play or where you end up at the end of the season, there's no reason for someone like Rakitic to accept lower wages in a worse team, he can either stay at the club and end up playing ~2000 minutes this season earning more than in Manchester, or leave to a better team than United while keeping his wage at Barcelona.
He's on ~160k por week without bonus, you're talking about 130k and god knows he won't get a bonus for winning the UCL or the EPL, would you take a 20% cut in your wages to work for a worse company?.
And it's not about his future playing time either, United have enough money to sign a better player than Rakitic this June, so he could end up earning less, living in a foreign country and riding the bench again in half a year.
There was yesterday a video by Tifo that said United's mean salary was 160k, you think Rakitic is worse than an actual average United squad player?.