Liverpool are trying to sign Lemar from Monaco ahead of tomorrow night's 11pm transfer deadline.
Arsenal were interested in the France international earlier in the summer before Arsene Wenger claimed the deal was "dead".
And Manchester United have also been linked with Lemar with Jose Mourinho keen on landing a late fourth signing.
But according to journalist Castles, Liverpool will have to pay far more than rivals United and Arsenal were quoted for the 21-year-old.
"Monaco's asking price when Manchester United and Arsenal were interested in the player was €60m (£55m)," Castles said on The Transfer Window podcast.
"That interest remains from Manchester United, but now they've got Liverpool on the hook and making offers the price is gonna go up beyond that level in a classic Monaco 'you want one of our players we are gonna squeeze the maximum amount of money out of you.'
"Which they've done better than any other team in the history of football this summer."
Thomas Lemar skies shot and takes shockingly bad free-kick
Monaco have sold Bernardo Silva, Benjamin Mendy and Tiemoue Bakayoko to Manchester City and Chelsea respectively for a combined fee of around £135.
And wonderkid Kylian Mbappe is set to join Paris Saint-Germain in an initial loan deal ahead of a £166m move next summer.
We use your sign-up to provide content in ways you've consented to and to improve our understanding of you. This may include adverts from us and 3rd parties based on our understanding. You can unsubscribe at any time. Read our Privacy Policy
