This app was mentioned in 3 comments, with an average of 2.33 upvotes
Using the "Airsoft Resale Calculator" app (based on /u/Vorpalbunnie's resale formula), and assuming minimal usage, the absolute most they would be worth is around $440 total, which is approximately 70% of their new value. Again, that's assuming that they are basically brand new, but still used.
I'd agree with /u/MrBlackMaze though. They likely have more sentimental value to you than the amount you would get from selling them
There's a free app on the Google Play Store that's actually based on his formula (/u/Vorpalbunnie is even mentioned in the description)
https://play.google.com/store/apps/details?id=com.natech.airsoftresalecalculatorfree
If you're on Android, there's an app based around that formula