How much for an island?

As president elect Trump has suggested multiple times (no matter how mental it is) that he wants to buy Greenland. I don't agree with him and I don't want to get into the politics or him as a person. For purposes of this post just forget about him from here on in.

Supposing another country wanted to buy a Scottish island. Which island would you consider punting? What price would be acceptable to you? This could be money, infrastructure, or simply a cash figure in everyone's bank account.

What would be the big risks in selling an island? I'm guessing fishing and mineral rights are the biggest consideration.