Pretty much any time you trade anything in you get the short end. They are not going to give you full price because they in turn are going to turn around and sell it. If you trade it in you are doing so for convenience so you don't have to worry with the sale but you will come out on the short in most times.
I dont worry with a back up bow unless I was going on a trip somewhere. For the most part you can get your main bow fixed fast unless something catastrophic happened to it. Plus I doubt you would shoot it much and keep the string changed and such so that it would be ready on a moments notice. I mean really, just how many people do you see using their back up bow?