I've got a handful lying around, and honestly, the prices are totally dependent on condition and region. Pokémon and Tetris are the classics, sure, but with Pokémon it really depends which edition - the German versions go for less than the English or Japanese ones. What a lot of people miss: it's not just the original Game Boy games, but certain Color and Advance titles can be seriously valuable if they were rare. The Legend of Zelda: Link's Awakening, for example, still costs a decent amount secondhand.
The catch is you've also got to prove that they actually work. A broken game is basically worthless, no matter how rare it is. If your cartridges still run fine and the labels are intact, I'd check eBay or platforms like Whatnot to see what your specific titles are going for right now. Some lesser-known stuff like certain Japanese RPGs are sometimes sought after more than you'd think - some of them bring surprisingly good money.
Is it worth collecting anymore? More as a hobby, if you're into it anyway. As an investment? Eh, the days of astronomical prices are over. But you don't have to sell them quickly either - the games aren't getting any younger, and people with nostalgia will still pay decent money for them.