I've built my old, el-cheapo frame up into a single speed hack bike for local errands and the like.
Mostly parts I already had knocking around. The wheels came from freecycle. Main expense was the parts required to convert to singlespeed - chain tensioner, sprocket, spacers, chainring bolts.
I was trying to build it for as little as possible and annoyingly I bought a new seatpost and then found I already had one and I could have sawn off my old handlebars and used them. Didn't think about that until I'd already hit the buy button.
Still, only cost £60 all-in.