This is true, but a good starting point is here:
http://www.transfermarkt.co.uk/en/john-guidetti/profil/spieler_98596.html
This has proven to be a pretty good measure point- if a club doesn't want to sell, the player will cost around twice this value. In extraordinary cases like for Andy Carroll it can be as much as three times the value. I'd imagine we could get him for around 4-5m
If it was 4-5 million I would be delighted. Really depends if Man City still want him, I imagine as he only 20 they would keep their options open for a season or 2 more.



