Your best bet would to probably ask around on Simpsons merchandise sites. There probably are a few out there for extreme collectors of all Simpsons merchandise. We don't generally guess at how much something is worth here. We are a wiki and deal with facts that we are given from various sources so can't come up with an answer.
As for any other information on it, we don't have an article, nor a section of our Burger King article for the poster. A picture would be great for our wiki if you wouldn't mind but I'm afraid that you know more about it than we do.