It would take a huge amount of money, and even more so for a rival PL club. Also, Chelsea are already pretty stacked in midfield, so would be a waste of Mainoo's time anyway.
It's silly season, journos are taking the 'every player has a price' line and shit stirring with it. I think Mainoo might not be the second coming of Robson or Keane like we all wish for, but he's already very good and will probably continue to improve.
That costs a lot of money to replace, especially when you factor in how young he is and so could therefore be around a very long time still.