Heartgold/Soulsilver is crazzzzzy expensive and has been for a while. Not sure why that game specifically. Did they not sell a lot of copies maybe? It sucks cause I've never played gen 2 and would like to but not for $200+ for a DS game. I can emulate but I like the idea of a physical copy.
I remember trading my HG/SS and other Pokemon games in a couple years ago and HG/SS had like, $65 trade-in value at GameStop at the time, which surprised me. But it's also the only way I could have gotten my Switch + Smash Ultimate for only $12 out of pocket. I made out like a bandit, lmao.