$80 is definitely a high price. If you buy a GBC from eBay or Craigslists etc on any given day I’d expect to pay around $80.
However, if you’re monitoring market places like Facebook or Reddit, you’ll come across them for like $30-40 by people who just want to get rid of them. It might take a few days of waiting to come across one but it happens regularly you just have to be quick and have multiple payment options ready.
I’ve bought a few GB-GBAs recently and I’ve never paid more than $45 for one. I probably average $30 or $35 because I buy multiples.