nothing like it on net to compare. just think its so unique and a bit creepy cause they shared the same fate. its a Hollywood diner in Perth, Western Australia, I bought it a couple of years ago when the restaurant went under. I have no idea of its value, any educated guess or evaluation would be appreciated
If the signature is clear (and says "Heath Ledger" instead of "Heath" or "H. Ledger") and it is certified genuine, I'd think about selling it as soon as possible if you want to make money from this. I doubt Heath Ledger will be as well known in fifty years as James Dean is now, hence the value will drop.
As for what it's worth? It would certainly interest a collector and any back-story that can be attributed to it would push up the price. Like you say, it's interesting that it was James Dean. If you get very lucky and find someone who is really looking for this kind of thing, it could attract a large price, but I'd be surprised if it got into treble figues.