$600. You can see my ad on craigslist here: http://grandrapids.craigslist.org/mcy/4484330529.html . It's a good bike and it runs straight down the road. Just needs a little work to be "restored".
Here's my description from craigslist:
Up for sale is my 1978 Yamaha Chappy Moped. It is in good running condition. It goes and stops as it should and goes up to 25 mph with a big guy on it (me). Battery needs a charge, but was new last year. All electronics work when battery is charged. Could use some work to be "perfect", but it's a good bike. Newer tires.
No lowballers. I know what this bike is worth. If you're going to offer me $400, don't call. I'll keep it if the offers I get are too rotten; I have no need to sell.