Fee is just one part. You have to look at the overall package when you are signing a player to get the actual cost.
A 42m few + 250/wk (assuming) for 5 years makes it a pretty steep deal. That's 13m/annum in wages. 65m over a 5 year contract. That's over a 100m in costs for a player with plenty of red flags.
I agree, however:
Who are the alternatives that are as good or on a similar level as de Ligt, would cost below £50m, are the optimal age, and would also accept around £100-120k per week in wages? We aren't saving that much money if we pay more in transfer fees, but lower wages.
Lowballing every club for their players and not willing to match even their fair value, be that the transfer fee or wages, will just have us end up signing 10th choice players in every position. Not overpaying isn't equal to not even paying fair value. That would be going from one extreme to another. Whenever there's a good opportunity, sure, take it, but we're Manchester United, one of the biggest and richest clubs in the world. Our wage bill should be among the highest ones in world football, and we can't solely rely on just finding bargains all the time. Elite talent costs a premium, and avoiding them makes it impossible to reach City's or Arsenal's level. I don't want us to be like Liverpool and miss out on the best players, because "value in the market" and Klopp was enough to usually stay in the top 4, with the occasional title challenge. If we want to get back to the top, our recruitment policy has to be an optimal mixture of different types of deals. Finding bargains is always great, but you do have to occasionally, I'd even say regularly, pay big money for the best talents out there. Why do you think City wanted both Rice and Bellingham in 2023, when they were coming off of a treble winning season, and also already had Rodri? Because 1. they lost Gündogan 2. they wanted to stay on top...but this isn't even that relevant in regards to de Ligt's situation, as 42m isn't a premium fee for him, it's rather probably significantly below his real market value.
Regarding the wages, if Casemiro leaves, we'll only have 6 players on more than 150k/week: Rashford at 300k, Mount at 250k, Fernandes at 240k, Antony at 200k, Sancho at 195k, and Maguire at 190k. The future of some of these players is also uncertain. Varane (340k), Martial (250k), Ronaldo (515k), De Gea (375k), Pogba (290k), Cavani (250k), and Mata (160k) have all left the club in the last 24 months. Casemiro (350k), Antony (200k), Sancho (195k), Maguire (190k), Eriksen (150k), van de Beek (90k), and a few others on similar wages as the latter two might also leave. There's a lot of space on our wage bill right now for a player of de Ligt's quality, even if he's going to be on 200k/week or more.