It's been a gradual process of changing things on this bike for the past year. I bought this bike used in the early 1990's for $150. It's a 1980's Specialized Hardrock. Last summer, the gears started slipping. The drive train was worn out. I bought a pre-built rear wheel with a Surly hub and Sun Rhyno-Lite rim from my LBS. I have ridden this wheel for the past year as a single speed. The crank was a cheapo rivetted POS. I bought a BMX crank, Surly chainwheel, and Shimano BMX freewheel. I have been riding this for the past year. The original SS configuration had a 35 tooth chainwheel. I have changed it to 38t now. This bikes horizontal dropouts are short. Not all combinations of chainwheel and rear cog work because adding or removing a chain link moves the axel 1/2 an inch.
I'll count the cost without the extra stuff I bought while playing around with SS and different gear ratios.
$140 wheel
$50 crank
$35 chainring
$20 cog
$15 chain
Total cost to "fix" my broken bike was $260. Remember, though, that my bike didn't work before this project. A friend of mine with an almost identical bike had the LBS replace his worn out drive train with new cheapo parts (identical rivetted chainring). He paid $200 for the repairs.
I also bought new handlebars and a new seat, but that wasn't necessary for the fixed gear conversion.