What gun could possible be worth 50 or less?
Gun amnesty programs get a lot of weapons that didn't belong to the person turning them in--thus it's worth turning in because the person doesn't have legal custody of the weapon in the first place. Examples--grandma takes a gun out of her gangsta grandchild's room, or widow turns in dead's husband gun that she never wanted in the house in the first place, or couple turns in gun found in the house they just bought. Sometimes they get WWII weapons soldiers, for some reason, brought home with them from war, and now the grandkids have. Or someone decides to turn in a weapon they purchased illegally.
It gets another gun off the street. Check out the stats as to why law enforcement feels it's a successful program--they've been doing it for years b/c of the measurable + outcomes.
It's voluntary.