They're only worth what someone will pay for them. Upgrades are a slippery slope, some will pay more, some won't, it's a crapshoot, so do as you like and don't worry about it, is my opinion.
I usually do a lot to my airguns, if they turn out special, then I keep them for a while, if I change my mind or don't like them, they're sold, for whatever they bring. I accept that.
Look at the Brocock Sniper, I have an HR and it is one ROCK SOLID rifle, but if you go to sell it, you'd be lucky to get $800 out of it, and the newer XR sells for $1500? I love mine, it's not the most powerful, or the lightest, but it just works, every time. So, it stays. I buy almost all of my guns used, and if they need rebuilt, then I do it. I've gotten most of them for a good deal and they're served me well. I don't give much thought to resale value, until someone mentions it.
The market does seem stagnant right now, and I'm not sure why. I think, at least, a lot of the prices in the classifieds are reasonable, compared to new. I admit some are out of line. If you see a used gun listed for only a hunderd or so off new, it's going to have a hard time selling. Fact of life. Me, I believe in trading, then you can get a deal on both ends. I may not always get the gun I'm looking for, but sometimes, as in the case of my Cricket Carbine, you get much more than you expected.