The answer came in the recent push to get my kids to treat their rooms like places they want to spend time in, as opposed to trash heaps that happen to have a bed. They both have been clearing out trash and donating clutter at a slightly terrifying rate. (How can they throw out so much stuff and still have a full room?) We have a brick store locally, which is a place that buys, sells, and trades used Legos. The kid gathered up a huge tub of Legos and asked me to take him to the store to sell them, so we planned on that for this evening. I seriously had no idea how much money he would get--two dollars? Fifty?
The guy at the brick store, long-haired and lanky, sifted through the bin, then said, "You've got about four gallons here, but a lot of non-Lego stuff mixed in. I can give you three bucks a gallon, so that'd be $12."
We looked at each other and shrugged. "Sure," said the kid.
"Or," the clerk continued, "if you want to sort through it better, I could offer you more money. You probably wouldn't have as much left, but it'd be worth more per pound."
So there we sat on a bench in the small mall, pulling out debris and non-Lego pieces from the bin. After I held up what I took to be a small ball of foil, but was actually a helmet, he started teasing me. "That's a LEGO, Mom!" he'd say when I held up a broken pencil, a Nerf dart, a scrap of paper.
He (we) ended up getting seventeen dollars for the cleaned out box. I didn't mind helping, since I figured it was less I'd have to pay also. Then we found a twenty dollar skateboard at the store, and celebrated by going to a breakfast place for dinner. The waitress mentioned several times how handsome my son is, and I wanted to ask her if she thought he was in charge of the tip, but he was adorably flattered by her kindness. "She was really nice!" he kept saying. I left a solid 20% after all.
I have no idea how long a twenty dollar skateboard will last, but his delight in it is, as the ads say, priceless.