What is your preference/General rule when it comes to collecting games?
I mainly like to collect complete boxed games in good condition. I don't have many loose carts in my library but i think it's better to find a cib game than to hunt down for a loose copy, box and manual. Saves me time, headache and probably money. But I will buy a loose cart only if buying it complete is too expensive. Earthbound would be an example.
If it's a sealed vintage game or used in very good condition/complete $50 is proper.. its like paying for the game in its original msrp price.. depends on the system though. Anything below $50 is a good deal. And depending on rarity/quality, common games shouldn't cost more than $20. $100 is the most I'd be willing to spend on a semi-rare/rare game. 5 Years ago, I got a sealed copy of Ys Book 1 & 2 for $80, thought it was a little too expensive at the time considering I was eyeing a jewelcase copy earlier but got outbidded for $40 something. Good sealed Turbo games are probably $100+ nowadays.
And depending on the going rate of specific games I try to save 20-30% of the game's perceived market value. Just recently bought a complete Tactics Ogre on PS1 for $50. It's currently averages around $70+ per sale. So I was pleased i got it cheaper than expected and I've wanted to get this game for years..