There was some grumbling from Old Trafford recently at what they saw as a 'United Tax' - selling clubs deciding to charge more when they saw how deep the pockets of the buying club are.
The transfer market is a curious thing given how much subjectivity there is in valuing players amid different circumstances around supply and demand.
To take two recent examples, United could drive the price for Mason Mount down because Chelsea were under pressure to sell, but Ajax could charge more for Antony last year because they had already sold players and there was limited time left in the market to sign a replacement.
Despite the unique pressures around deals, there is a framework by which clubs buy and sell. Each player can only ever be worth what the buyers are willing to pay and over time those decisions dictate how prices go up and down across a market.