Fees are set by highlight quality and age, while the trait that actually determines value gets priced almost by accident.
Availability is the skill the transfer market keeps underpaying
A player who gives you fifty starts a season is worth more than a player who gives you twenty-eight and a better pass completion figure. Everybody in recruitment agrees with that sentence. Almost nobody prices it.
The reason is structural, not stupid. A transfer fee is negotiated against a comparable market, and the comparables that circulate are goals, assists, minutes at a big club, and international caps. Durability is a negative attribute, which means it only shows up when it is absent. Nobody markets a player as reliably present. Agents do not build a pitch around a clean medical history, because the pitch has to be exciting enough to start an auction.
So availability gets handled at the wrong end of the process. It goes into the medical, which happens after the fee is agreed, and the medical is a pass or fail gate rather than a price adjustment. A club discovers a knee it does not like and either walks away or signs anyway with a slightly reworked payment schedule. The information arrives too late to move the number in any serious way.
Insurance is the one place where the truth gets a value. Underwriters will quote a squad differently based on injury history, and those quotes are the closest thing the sport has to an honest price for fragility. Recruitment departments almost never see them, because insurance sits with finance and finance is not in the transfer meeting.
There is also an incentive problem inside the selling club. A player with a difficult history is exactly the player you most want to sell while the highlight reel still looks good. The seller knows more than the buyer, disclosure obligations are thin, and the buyer has a deadline. That is a market designed to misprice.
I would restructure the deal itself. Push more of the fee into availability-linked instalments: appearances made, not appearances possible. Some clubs already do this in a token way, twenty or thirty games triggering a payment. Make it the spine of the contract rather than an afterthought and the incentives on both sides start to align.
Until then, the cheapest edge in recruitment is buying the boring player who never misses. He will always be underpriced, because nothing about him makes a good video.

