What would the the average price be for this coin in BTC?
Such a coin is a collectible. And the people from
Collectibles subforum can answer best to this answer or point you to the right direction.
And since many of them don't have other interests in this forum, best is to ask
there (either move this topic there, either ask again)
Is it worth putting it into a crypto exchange and earn interest on that, or should I keep it closed?
Any proper bitcoiner will tell you that it's not worth to move coins from your wallet to the custody of somebody else, no matter how reputed that somebody else is. Shortly "not your keys, not your coins".
Bitcoin, on the average, has a very good increase in price over time, so seeking extra earnings has more risks than benefits.
In your case, since it's a collectible, also peeling off the seal for the coin should decrease its value by a great deal.
But again, I am not a specialist in this, go to the collectibles section and ask there, it's the best approach:
https://bitcointalk.org/index.php?board=217.0