keeping an expensive addiction cheap :-P

Upgrade your guns is probably the cheapest.

I am a little confused by your post, so will just answer what I interpreted from it.

Trade-In wise - I don't know if any stores are doing that anymore, and the guns you have mentioned are a dime-a-dozen, so wouldn't be worth much of anything as a trade-in. So that's chasing after a dog that's lost its bite.

The cheapest and most cost-effective way of staying involved in this hobby is to learn the craft of tech-work. If you can expand your understanding of your guns, and how to tweak them, then you can buy relatively cheap (in price, not quality) parts and improve your guns performance to no end.

If your guns are quite old then they need a complete overhaul. Strip the gun down, and get the gearbox out of the body. Gut the gearbox, and wash off all of the caked-on Chinese grease. If you've got nylon bushings, replace for metal ones and glue 'em down. Inspect your gears for signs of wear - if they look alright, then stick them back in and re-shim. Shim to the pinion gear, NOT the spur. It will run smoother, quieter and be far more reliable in the long run. To do this just attach your pistol grip (on a V2; motor cage on v3) to one side of the gearbox shell, and put in the motor. Adjust motor height for the most contact/best meshing of teeth, and shim the bevel gear accordingly. Shim the other gears to get the best meshing of teeth, and take your time! Better it take you an hour to do by having patience, than having to redo it six or seven times.

Inspect your switch assembly - what are the wires like? Are they bent, and thin and degraded? If so replace with higher quality wiring. What are your trigger contacts like? If they're blackened with carbon, clean them up and have a look. If you don't want to solder new wires on, but do want to replace them, then buy a new switch assembly - it'll cost you €20. While you're at it, get a basic MOSFET (i.e. build it yourself); costs next to nothing, and improves the lifespan of your gearbox.

Have a look at the small springs in your gearbox - these can be replaced for as little as €3, and if they have been kept under tension for a while (i.e. tappet plate at rearmost position), then I'd replace 'em. For the sake of a couple of euro.

Inspect your piston - if there's no sign of wear, put it back in. Chinese pistons (such as CYMA etc) are actually very durable; I wouldn't worry there. Replace the piston head O-ring though, and lube it up generously. Check compression on your cylinder with air nozzle attached (at rearmost position, and again at its foremost position). If your compression is good i.e. feeling strong resistance then put in an M85 or M90 spring (Guarder springs are stronger than they're rated, so use a different brand or lower rating). If compression is poor and there's drilled parts (nozzle, cylinder etc), then replace the air seal parts (cylinder, cylinder head, air nozzle) with parts of the same brand. I am a big advocate of staying within the same brand for compression parts as they're usually made to match their own tolerances i.e. superior performance! While you're looking at your piston/air seal - have a look at the AOE (angle of engagement); you want the pick-up of the piston to be smooth with as little strain as possible. Having the piston pick up at just beyond the 12 o'clock angle as opposed to 1 o'clock will drastically reduce blowing the arse out of your piston. Adjust it using sorbothene pads glued onto the cylinder head. This reduces power slightly, which is worth noting.

Grease your gears, lube the cylinder, and reassemble. Test the gearbox and ensure it's running smooth, and then move on to your barrel and hop up. Install a good 6.04mm or 6.03mm tightbore barrel, and a soft hop up rubber - reassemble rifle, and chrono. Test for range and accuracy, and adjust parts where necessary i.e. change barrel, hop up rubber, spring etc to ensure it is firing right.

Sounds like a lot of work, and it is. But if you do it, it's free. All you have is the cost of parts:
Gears - €20
Air seal parts - €10-15 (cylinder, cylinder head & air nozzle)
Piston - €10
Springs - €12-15 (tappet plate, trolley, trigger, safety bar, cut off lever, main spring etc)
Switch assembly - €20
Basic MOSFET - €15 (cheaper if you build it yourself)
Bushings - €5
Barrel - €40
Hop up rubber - €5-10

You're looking at a total cost of circa €150 if you had to replace virtually every part, and you won't have to do that. Your gears should be fine, switch assembly should be serviceable, and it will likely already have metal bushings. If the gun has been drilled it is recommended to switch out the air seal parts, but not essential. MOSFET is not a prerequisite, but it does help. Barrels and hop up rubbers improve performance of the gun, but are not essential from the word go. You would likely just be replacing the piston, cylinder, barrel, hop & spring costing in and around €60, and giving your guns a new lease of life.

That is, in my opinion, the best way of keeping it cheap