That's not just Gander Mountain. A gun shop isn't going to give you high value for a trade in. They intend to make money.
If you want more money, sell it yourself.
For awhile Gander was my go to place to sell stuff as they did offer decent sell/trade prices which eliminated the hassle of selling it myself. After awhile though they went downhill and seemed even worse than most LGS's. I was a buyer for an LGS so I know the deal. But when they tried to offer me 300.00 for a like new Browning BT-99 that they sell for almost 1200.00 I knew that they had lost their mind.