I paid $25 for a Schwinn CrissCross that had been converted to single speed, put some spare gears and shifters on it and sold it for $120. I like that kinda deal!
Now I'm taking the single speed cog and putting that on a butchered old Schwinn Varsity I got for free -- minus the derailleurs -- and will sell it as a single speed.... Fun stuff!
I grew up riding single speed bikes and once I discovered the joy of shifting gears, I never looked back!!!