Find an old rigid MTB frame and build it up. You can strip it down for nothing, a rear wheel can be built for less than $80, and you're in business.
Here's my singlespeed which (when I'm cleared on post-op to start riding again) will become my commuter again. It's a 1988 Trek 400 I pulled from a dumpster and dropped a 1991 Shimano 105sc crank onto (with a single ring). I built the rear wheel with an IRO high flange fix/fix hub and the whole thing runs on a 44/18 gear for 65 total inches of hill climbing power. I'm splitting the fenders with a pair of reacharounds to clear 30mm CX tires for the winter.
I have a 24 - 30 mile r/t commute, depending on the route; including some long climbs between 5 and 8% grade.
