My conversion was my dad's old bike. he mailed it to be complete, so all i had to pay was shipping ($36, but that included an extra frame, which i could easily get $60+ for.

$bin - seatpost (prev bike)
$(gift for building a friend's bike up)brooks - saddle
$on bike - bb
$traded a wheelset for a set of 105 cranks - crank
$5 for first ones, they blew and bought Hellyers, $50 - pedals
$155 - wheels
$8 - chain
$30 - freewheel/cog(s)
$on bike - stem
$on bike - headset
$chopped bullhorns on bike - bars
$3, cloth tape from co-op - tape
$traded - levers (if applicable)
$from bike - cables and housings (if applicable)
$on bike - calipers (if applicable)

also, $50 for chainring and another $50 for tires.

a lot of parts were actually traded, such as the original crankset, wheels, all cables, shifters and cable housings. i listed the values that would have been charged to any old person that would drop by the shop.

didn't have to buy any tools for the project as i work at the co-op and am really good friends with the guys at the lbs, and between my accrued stand time and a few beers at the lbs, i didn't have to pay anything for labor or tool usage.

so, my total for my conversion was $382. it rides a lot better than some "track frames" i have ridden, and it is my belief that certain parts are very important to how a bike "feels". don't skimp on the drivetrain, its 90% of what makes a bike feel solid (im including wheels). it is currently in its second iteration, the first month of riding it i had less than $150 in it. once i decided i liked it, i bought a wheelset, an everwear, dialed in my gearing (tried 4 different chainring sizes, (48, 39, 45, and finally settled on a 42, only paid for the 42 (got to love used parts bins and friends). my cog has always been a 17 tooth. in the not future, i don't see changing out any of the major components, although i might trade the bars out with some other ones i have lying around, just to change up the feel of the bike.

It's easy to purchase a complete used bike (Road or MTB) in decent condition for under $200, and likely a lot less. Then you remove the derailleurs and shifters; if it has a freewheel hub remove all but one cog from the body and if it's a cassette hub leave one cog and add spacers; remove the large chainring (File down or buy new chainring bolts); if it's an older frame with long horizontal dropouts simply shorten the chain and if it has vertical dropouts put the rear derailleur back on and use it as a tensioner (You still need to shorten the chain); check the chainline; and go riding.

I've done this five times and within a year after starting to ride each one sold it and started anew. They were all nice steel road or mountain bikes and were spiffy, kick-around-town single speeds. I know I had a less than $600 total investment in all five (About $120 each and the least expensive was $70) and all of them are still being ridden.