I don't know what Atari 800 games are worth, but I'm not sure these old carts are exactly a goldmine anyway. I saw a list of Atari and Coleco cartridges, and I had quite a few that were considered "ultra rare." They could net something like $35. To me, that's not worth losing cartridges I've had for years. I'll hold on to them.
Regarding Atari carts, yes, they have not appreciated that much. But in about 40 years there will be far fewer working ones around. But I won't be around either. Told my daughter to save them.