It really depends on what I'm after. For new systems ('new' in this case meaning 'readily available at any Gamestop'), probably right around that $20-30 range. Once in awhile I'll spend more if it's a title I don't expect to see a price drop on for awhile- for example, I used a coupon to get Mario+Rabbids for $35 just a couple weeks ago.
For older systems, the what & where factor in heavily- I'll balk at a yard sale price that I'd gladly pay at a reputable local game store, for example. I'll go a little higher for a specific church rummage sale held every summer, just becuase they gave me my money back when I foolishly didn't check a case & discovered after leaving there was no disc. It was a good hour or more later when I came back, they had no reason to believe me- but they did, and that counts for something.
Ebay is where the highest prices end up, just becuase that's where I typically have to turn for the really obscure/rare/expensive stuff when I'm willing to pony up for something. Right now the record holder is $320 for a complete Magic Knight Rayearth on Saturn- but that's a real outlier. I'd wanted a Saturn since I was 13, and that was one of the original games I'd wanted for it. I was selling off some unwanted old DVDs, found out a few were valuable, and flipped the money into the game. Typically, if it goes for over $50 on Ebay, I'm not likely to get it in any real hurry.