Your argument suggests that the market rate for a player of Sanchez's quality is higher than £50m (or else you would clearly conclude that you could replace him for £50m). But that is to ignore how long he has left on his contract. Yes, the value of a player of his quality on a 5-year contract may be higher, but you don't have him on a contract with 5 years left to run.
You should look to replace him with someone with a year or less to run (how much did Ibra cost United?) or spend more than £50m. Frankly, for a club based in London - one of the richest cities in the entire world - with an enormous stadium and some of the highest season ticket prices in the world, playing in the richest league in the world, and who's just benefitted from a huge injection of cash from BT (as we all have) to be bleating about transfer fees, shows the lack of ambition and/or tightness of Kroenke more than anything else.