Also at least right now using GTA 5 as the example the price is $39.88 on Amazon for a new PS4 copy. Gamestop.com has the same game for $59.99.

So as a prime member you can save $20 on the game (20% of your prime membership) while non-prime members can't buy it for any price. I agree that the discount should be prime exclusive but for now it seems to be the Costco policy.

Yep, looks like UK only. just the specific versions like the GTA V PS4 version on the US side.