The cost of a conversion depends on one's abilities but of course there are compromises no matter what route you take. I built my first FG around '84 and all it cost me was the price of a cog and fully functional beater... it was a Schwinn of some sort. I'm sure you know the routine of stripping the unnecessary crap so I'll spare you the diatribe and focus on the rear wheel. All that was needed was to remove the freewheel, spin the cog on, loosen the axle so that I could shift most of it over to the non-drive side and set the wheel into the frame. Then I began dishing the wheel so that the chainline was correct and the rim centered. Once this was accomplished I made up the appropriately sized axle spacers from nuts, spacers and washers lying around from 'donor' bikes/wheels. Once the rear wheel was properly dished and true I filed down all spoke ends that may have been peeking above the spoke nipple tops otherwise I would have had frequent flats.

It may be a good idea, when using a road hub, to just be safe and epoxy the cog onto the rear hub... you don't want it loosening up if you are inclined to do skid stops.

This is a cheap way to build a fixie and I still build my beater commuters this way. Sure the rear wheel is basically rendered disposable for anything other than as use for a fixie but then the better methods do cost more.