I am pretty sure it is based on store credit. One card that I looked up on the GS website had a trade in credit for $89.31. If you are a GS pro member, then you’d get 15% more for $102.71.
The trade in credit when I sold it to Gamestop was $88.31, but they subtracted 10% for cash so the cash value was $79.38 (This $79.38 is the value the GS employee quoted to me when he asked if I wanted to sell). At the same time, I still got the 15% extra for being a GS pro member so the actual cash value I got for the card was $92.44.