This is a complex issue that rally has no straight answer, and probably one that the answer changes with what you are talking about.
Some things you can look at and answer quickly because we can compare them to real items. Other things, we have no real world comparison, and trying to "reverse engineer" or even try to compare similar items doesn't work. (Am I am guilty of this all the time too)
Take the question of "I want my pistol to carry more ammo so I don't reload as often". Depending on the weapon, the solution is easy: "Buy the magazine upgrade mod". Some times its not so easy (like revolvers), and the solution is a whole new item (an automatic). Now, does that mean that automatic costs: [Automatic Purchase price]-[revolver purchase price] =Cost +30%?? Or does it mean he has to buy the automatic and later sell revolver for whatever he can get?
Now why am I bring this up when this is about equipment?
Because they are related. A gun is still a piece of equipment, just like a commlink. And Just like there are many guns, there are many commlinks.
Why would a Business, that is in the market of selling goods, buy a USED item from you for the same price they sell a NEW one for? Yes, they get to make a sale of a more expensive item, but they now have a used item that they have to sell for more then a brand new item to recover their costs. And that's assuming you can find a dealership that is willing to buy your old stuff in the first place. And then there is the "cross brand" thing to worry about. (Chevy isn't going to give me the $65,000 I paid for my Mustang if I want to buy a Corvette.)
Sadly, when it comes to the purchase of a new piece of equipment, it's what the equipment costs, modified by any regional pressures. They are free to sell the old equipment to whomever they can find to buy it however. (standard fencing rules).
But keep in mind some equipment can accept modifications which usually cost a fraction of the price of a whole new device.