For ebay prices are sometimes meaningless unless you filter by "sold items". But yeah looks like they're going for around $100-$170

Price Charting has it broken down by type. That site isn't 100% accurate by a long shot (notably it doesn't really factor in condition), but it's helpful for a rough idea. I saw it had the complete game at over $400.

I'm guessing the game is popular with collectors for being a "rare gem" of the Game Boy library (since it's both rare and actually a pretty good game). Clicking around, I see it in some rare Game Boy Color game lists—along with Shantae, which I strongly considered picking up back in the day from Electronics Boutique for like $20 and decided "nah."