Extebarria's contract expires in 18 months, a fact that has reportedly led the Premier League clubs to prepare a cut-price offer for Betis.
However, the Verdiblancos coach has insisted that such bids will be rejected, and that the midfielder will not leave Seville unless his minimum fee release clause value is met.
"Benat's clause is €20million and he will only leave for €20million," Mel said.
"To sell him for anything less would be a disaster. The club must defend its position and Benat knows that. To be able to go the market and find a replacement, the only number is €20million.
"He has one more year left on his contract and he can fulfill that."
Meanwhile, Mel also said that he did not expect that Jose Canas - targeted by Swansea would leave before the summer.
Canas is widely reported to have already agreed terms with the Swans who are prepared to do a deal now - but Betis are standing firm.
"I am 100% sure that he will not stay here after June 30," he said.
"Knowing his commitment to the club and his teammates, he will give 100% while he is here. (On Friday at Real Zaragoza), he will play."