If you think ideas are worth something, I have a huge pile of them I will gladly sell you.
Now, an idea + something else can be of value, but then it is beyond a mere idea. An idea which has been validated by market research to show its marketable and at least enough technical research to give a rough idea of development costs and production costs can be valuable, but then you have added a lot to it.
Even just an idea + a detailed plan for researching and then developping the idea can be valuable, but again that has at least added something.
I won't quite go so far as to say that a bare idea is worthless (absolutes often make me hesitate), but without something more it is worth very little.