Derby boss Nigel Clough is leading the chase to sign the 21-year-old right-back, but Thordarson is keen to hang on to a key squad member.
He said: "There is definitely interest from Cloughie, because he has spoken about him on quite a number of occasions.
"I don't want to sell him because John is going to be a key player for me. Every player in the world has a price but we won't be selling him for anything less than what we think he is worth.
"I don't want it to drag on until the eve of the season and then in a flash of smoke he has gone because he would be another influential player I would have to replace."