Well running a suicide hub would be the cheapest and as long as your using brakes it would probably be fine, but you won't get the full effect of fixed riding if you can't apply back pressure without unscrewing the cog.
Take a look around your LBS for a cheap fixed wheel and if they don't have any ask how much they'll charge to relace your rim to a new hub, worst comes to worst you can put a freewheel on a fixed threaded hub.