With it's low value if it were me I'd keep it. Never know when you might want to use it. If you're going into a particularly dirty or rough situation it might come in handy. Or sometime when you decide you want to use 2 bodies to shoot with.
Keep it. Even if you don't normally use it, it's there if you need it. You will have it if you send your current body in for service, or if you don't feel comfortable using your normal body in a situation around water, sand, etc.
I did upgrade my D40 to the 7100 almost a year ago and I did keep it for a second body. The D7100 carries the 24-70 2.8 and the D40 carries the 10 - 24. This combo works very well for me in the parks and at home.
Save it, for your kids or young family members. They might need it for a school project. Or, it may spark and interest or hobby. That's worth more than $50.