Weapons- All weapons you buy you own. That means even if you buy a brand new gatling gun to replace your old light rifle, you still own the light rifle. You can keep it for any reason, or you can trade it in for a discount on the new weapon. The discount is the old weapon's original price/4 (say a light rifle, $20,000/4=$5000). When buying a new weapon, you deduct the sum of the old weapon/4 from the new one's price (say a gatling gun. $70,000-5,000=$65,000). As the guns get bigger, better and more expensive, the bigger the trade-in will get.

Armors- When buy new armor and upgrading for all, you are trading in. That means you deduct the cost of the old armor/5 (say standard light. that's $20500/5 {$4100}) from the new. Using the example I just did, lets say your upgrading to standard medium. That's $32,000. So, the price of the trade in is ($32000-4100=$27900) $27,000, less then what you would have paid, but not by much. When its tougher, more expensive armor, you'll get bigger trade-ins.

Propulsions- When buy new propulsion and upgrading for all, you are trading in. That means you deduct the cost of the old prop/5 (say standard light. that's $20000/5 {$4000}) from the new. Using the example I just did, lets say your upgrading to standard medium. That's $38,000. So, the price of the trade in is ($38000-4000=$34000) $34,000, less then what you would have paid, but not by much. When its faster, more expensive propulsion, you'll get bigger trade-ins.