West Ham are said to be prepared to go as high as £25m for the centre-back, but Carvalhal - appointed head coach a fortnight ago - claims that it will take double that sum before Swansea consider cashing in.
"When I signed it was very clear we would try and bring in players to improve the team," he told reporters. "We will not sell the best players in January, the message is clear.
"I have had guarantees from the chairman, but it depends if someone comes in and gives £50m, £100m, £250m. Of course, then it is completely different - to me, [Philippe] Coutinho or Neymar.
"You can never say that it will not happen, but the fundamental idea is not to sell players and try to improve the team."
Mawson has started all 22 of Swansea's top-flight games this term, while also being used three times in the EFL Cup.