let's face it, these are just like most cars; as very few gain value. Most are just made to be used (driven), though some may give more enjoyment than others.
Drive a car off the lot or walk out the store with a new gun (or near anything else), and it's value will drop.

Trying to speculate on what may give the highest return? That may take a mix of knowledge and luck.... who's to say what value one of our Shields may hold years down the road when it's handed down to a child/grandchild (age appropriate lol).