I have a boxed sealed LEGO set from 2002. I attended the Children's BAFTA dinner, with several celebrities. Each had an exclusive Bob the Builder set as a gift under their seat, which included a LEGO BAFTA trophy and a LEGO Bob the builder scene. Pretty sure it's rare. At the time I also got it signed by Neil Morrisey who does the voice for Bob the Builder. I gave it to my godson. It's still in the attic. Wondered whether it's worth much. Any ideas? Never found it on the Internet.
For something like this, there will not be a standard price. Really, it would depend more on the number of people looking for this. Post it on ebay, see what you can get.
If the set was truly exclusive, the price is subjective.
Chrome-colored stormtrooper minifigures were given out at toys-r-us exclusively for a month or so, and depending on the buyer and seller, the transactions can constitute a trade, a dollar, or anywhere up to 30$.
I would suggest taking the advice of PsyLawn and getting it out onto the market if you wish, but keep in mind that the value most certainly goes up when the number of unopened boxes goes down. The signature might get you an upped price, though!
Thanks for getting the BTB theme song stuck in my head.