What would the the average price be for this coin in BTC?
Is it worth putting it into a crypto exchange and earn interest on that, or should I keep it closed?
Price depends on which year coin it is, what it's made out of, the condition it's in, and the current market sentiment towards these collectibles.
I wouldn't peel it to move it to an exchange for interest. Literally these coins are made so you don't have to store your BTC in some 3rd party holding solution. You'd be adding liabilities for very little reward.
I think when it comes to Casascius coins you have 3 options. Peel the coin and cash out the BTC (then sell the peeled coin). Sell the coin in tact for above the held value. Hold onto the coin until you're ready to cash out. Peeling to get low interest wouldn't be an option for me because you'd probably get more selling it in tact anyway (depending on your hold time with the interest), and you'd be opening yourself up to more liability. The only reasons I could see someone doing this is if you didn't trust Casascius anymore, or you wanted to diversify your passive income more than you want to invest in a collectible.