From memory the fee was £22mil with £5mil in additional clauses.
His book value today would be about £15.4mil as he wont have hit the clauses.
For an italian international 20 year old forward, playing and scoring for one of the richest clubs in the world then that sale carries a premium.
If its true that theyre making him an offer now then clearly its a priority for them to tie him down...that also should carry a premium.
He's 1.5 years into a 5 year deal and the length of contract hes under also carries a premium.
Since 00/01 season they have signed 15 players for more than €31mil:
https://www.transfermarkt.com/fc-paris-saint-germain/transferrekorde/verein/583
Comparing the players listed you'd put him in the €40-50mil range which is about £36-45mil.
Id be asking for £60mil based on the factors listed above but £40mil+ id be ok with as its £25mil on the book value profit.