The concept of "worth" is not objectively ascertained and can only be validated in hindsight.
We can only think about worth in the context of current market conditions and what we believe the total investment into the player over the life of that player's contract and what we believe his contribution will mean to United on the pitch and commercially.
I'm going to make up some numbers, so bear with me.
Say we pay a transfer fee of 80m and spend say 10m annually on his wages over 5 years. So, 130m over 5 years (let's forget about prospective resale potential) for an annual cost to the club of 26m per year.
But if instead get Maguire for 50 and assume the same 10m annual wages, we're really only shaving off 6m in total annual cost over each of those 5 years.
Sure, one always wants to save 6m if it's possible. But if the alternative to Maguire is Smalling or Jones, assuming we don't sell either one to partially offset the cost of Maguire, I'll go with Maguire, thank you very much. If we lose out on Maguire because a club which can bank on 600m in revenue isn't willing to spend the extra 6m over each of 5 years in the net additional cost it would have taken to address a critical need, we're out of our fecking minds.
Eat the cost. If there's a better central defender for the same amount of total outlay (transfer and wages), such as Khoulibaly, then let's do it. If there isn't, let's get on with it now so that Harry is fully integrated and ready to start for two tricky early season fixtures.