Reports say Manchester United have agreed to pay Leicester £80million for Maguire.
The England defender has been a target of both United and rival Manchester City throughout the summer.
If United do indeed pay £80m for Maguire, the 26-year-old would become the world’s most expensive defender.
It would break the current record of £75m which Liverpool paid Southampton for Virgil van Dijk in January 2018.
But Castles claims he has spoken to people at clubs in Europe who cannot believe how much United are going to spend for Maguire.
He says clubs of a similar size to United would not entertain offering that much for the former Hull man.
“I’ve talked to various people who are in charge of recruitment at European clubs who laugh at the transfer fee involved in this deal,” Castles said on The Transfer Window podcast.
“£60m guaranteed, £80m in total, relatively easy achieved bonuses I understand.
“The suggestion is there is no way a club of similar stature in Europe would buy Maguire for anything like that kind of money.
“And he probably wouldn’t make it into the national teams of a lot of the top nations around Europe.
“I guess what those experts think and what we think about Maguire and whether it’s a clever switch or not is irrelevant.
“In a sense we will find out.
“He’s going to be put into a Manchester United defence that will be fortified by [Aaron] Wan-Bissaka who is very good defensively and we’ll see how big a difference that makes to the way they play.”
Maguire has made 76 appearances in his two seasons at Leicester, scoring five goals.
He has become a regular in the England side under Gareth Southgate, netting once in his 20 Three Lions caps.