It's really hard to say exactly what you've got unopened. Most likely it's just an unissued 1903 barrel, such as the ones that certain companies are putting on old 1903's to refurbish them to like-new condition while still maintaining period correctness.
That being said, it's probably not worth more than a couple of hundred bucks, if that, considering that AIM Surplus recently had complete Springfield 1903's with brand new Krieger Criterion barrels for $750, and I've seen rebuilds with unissued barrels going for $900 or so. I'd say $300, tops, if you could find the right buyer.
Of course, this is all speculation. If you have something truly rare, all bets are off.