Value, like so many things, is relative.
A piece of bread is particularly valuable to a starving man. But not to someone who has just downed a double cheeseburger, chips and strawberry milkshake.
Thankfully, these days, the good people at Tesco and the Gap tell us how much it costs to buy a pint of milk or a pair of khaki shorts.
It saves a lot of time. The weekly shop is painful enough without having to negotiate the price of a six-pack of sausages or a box of lemon fancies.
But in football, there is no price list.
Perhaps there should be. "One striker, 28 years old, good quality, little bit of wear and tear around the edges, so I'll knock a bit off. That'll be £15 million. Do you want to follow me to the checkout?"
It doesn't work like that.
Football is still stuck in the dark ages, where deals are agreed to in an archaic barter system.
"Ooh, this is nice, I'll give you £6 million."
"No way, it's worth double that."
"No chance, you're talking rubbish. How about 7?"
It's like one giant Greek market where the stalls are filled with multi-million-pound footballers instead of locally sourced goat's cheese and knockoff Armani T-shirts.
Sir Alex Ferguson gave an insight in his autobiography about what it took to get Robin van Persie to Old Trafford from Arsenal in 2012.
The sticking point was that Arsene (Wenger) hoped to receive £30 million or more for his best player. It continues to drag on for several weeks, during which time I phoned Arsene two or three more times.
In time we all arrived at the point where Arsenal knew Robin was not going to re-sign and accepted that. Their options were Juventus or United.
Arsenal were trying to sell him abroad, but the player only wanted to join us. My understanding is that Van Persie sat down with Arsene and told him United was his preferred destination.
Our offer was £20 million. I warned Arsene we would never get to £25 million. Arsene was incredulous. He could not believe that Manchester United would refuse to stretch to £25 million for such a player. I told him again, I wouldn't go to £25 million.
A footballer is always worth what a club is willing to pay. In Van Persie's case, Ferguson and Wenger met somewhere in the middle and United paid £24 million.
Ferguson decided a 29-year-old striker, with a questionable injury record, was worth the money to prise the Premier League title back from Manchester City. And many United fans would argue he paid it all back last season with the goals that won a 20th championship.
If anything, during the course of United's title challenge last season, he was perhaps worth more than the £24 million United paid. Value is relative, and United needed Van Persie to win the league.
But this season, things have changed. Wayne Rooney's form had improved and Juan Mata has arrived from Chelsea. Van Persie is still important—he proved as much with his hat-trick against Olympiacos in the Champions League—but his value to United has diminished.
He's a year older. And the injury problems he suffered at Arsenal have resurfaced. But even fully fit, his relative value to United is lower now than it was last season. He's not doing things single-handedly anymore.
But that's not to say United should let him leave. Whoever is in charge next season will need the best players at his disposal.
And Van Persie, whether his value is now reduced or not, remains one of United's best.