you can always see trade in values for gamestop on their website, link above, or on the phone app. I've never, ever heard of any store adjusting values, trades are pretty automated through their register you just hand them the games, give them your ID/card, tell them you also want to pre order Call of Duty and put $5 of your trade towards it, and it all happens automatic. The website does NOT reflect bonuses, so for example the 50% bonus if you pre order Call of Duty would have to be added on top of any trade values, a $25 website trade in value would be $25 x 1.5 would be $37.50, and the 10% PUr Pro bonus is always off the base so $25 x 1.6 = $40.
the website is down now though so i dunno if any bonus trade values are still happening, the call of duty bonus was only over the weekend I think. There are some dudes that run around making hundreds of dollars in trade credit when weekends like this happen, like you can buy rory golf and minecraft stories at target for $10 each on clearance and trade them for $25-30 at gamestop, but the only thing you can really buy at gamestop nowadays is gaming stuff, food gift cards, PSN/xbox cards, or netflix/uber gift cards. They stopped allowing you to buy amazon, ebay and gas gift cards with trade credit last year.