The absolute cheapest way would be to steal a rear wheel with a track hub and a fixed cog and lockring on it.
The absolute cheapest honest way would be to get the freewheel off of that rear wheel, thread a fixed cog onto it, and Loctite/weld it on. This is unsafe and not recommended at all by anyone.
The absolute cheapest safe and honest way is to buy a rear wheel with a track hub and a fixed cog and lockring. Velomine is a good site for reasonably priced wheels, and you could also try your local craigslist and/or shops.