To be perfectly honest? No idea whatsover. The soundtrack CD can and has sold for around $100 on eBay though, so your chances of getting a high bid from somebody are pretty good.
The only problem is that people probably don't check eBay particularly frequently for such Grim Fandango merchandise, even though they'd snap it up in a second if they saw it. I highly recommend you set a pretty high reserve price, even though it costs a few dollars, so you don't end up having to sell it for something like $40 (which is massively undervalued) just because a high roller didn't happen to check eBay during the week you were selling it.
It's a rare item without any doubt. Although eBay has a lot of Grim Fandango copies, they're pretty much all the re-release ('LucasArts Classics') that came out a few years ago. They're certainly not limited editions, nor do they contain goodies.
The fact it's unopened is definitely going to add a lot of value, so keep it that way.