You put your project up for auction, and if someone thinks it's worthwhile they might throw you some money for it.
If your project is "cool", cheap to maintain, and non-profitable - then I think you can aim for something like $750 - $1500 for it. There are people who may see the potential in it, and would buy the codebase/brand to get a quick start.
You just need to calculate all your costs and sell it for the same price.