If you've got working parts already, take the shifters off, and use the rear derailleur as a tensioner and make it a singlespeed just for the purposes of picking a nice gear. When you figure that out, all you have to do is order a new wheel and drive train. On a double crank, the inner position will make a nice chainline.
Expenses: Wheel, cog, lockring, chainring, chain.
Things to check: Make sure you have forward facing dropouts--if you don't, you need an Eno eccentric hub. Make sure when you order your chainring, you get the right BCD.
I did this conversion to an old set of wolber/shimano 600 wheels that came with an old Cannondale. Depending on how much experience you have adjusting wheels it may take a while, but it was a really great experience.
I was able to do a full conversion from an old 6 speed to full fixed for the cost of a cog/lockring/chain. About $30.
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