Let me relate my conversion experience. I bought an old, incredibly rusty, Gitane for the conversion. I wanted the experience, but let's look at the cost.

With a good bit of work, I was able to reuse the handlebars, stem, brakes, bottom bracket, crank, headset and seatpost. I had a set of wheels with a freehub, tubes and tires, a saddle, pedals and a chain sitting around in my garage. I could have reused the brake levers, but I think old aluminum brake levers are too ugly to use.

The stem is permanently melded to the steerer, but it happens to be about where I need it, so I overhauled the headset with the stem in place, and I'm going to live with it.

Old, rusty Gitane: $40 (I overpaid)
Various cleaning products: $20
Grease: $4
Ball bearings: $5
Tektro R200A brake levers: $19
Brake cables: $15
Brake pads: $10
Bar tape: $12
Single speed conversion kit: $30

I got all this from local shops, so no shipping cost. First version of the bike on the road for $155.

I wasn't entirely happy with it like this. For one thing, the seatpost is about two inches too short for me. Beyond that, I like to upgrade. So I got a new seatpost and crankset. The old square taper didn't fit the new crankset, so the bottom bracket is going to have to be replaced.

Origin8 Pro Pulsion crank (cheap, but better than what I had): $50, shipped
Kalloy Uno seatpost: $19
Hardware for DIY tool to remove bottom bracket: $5
New bottom bracket: $31

So, version 2 will take me to about $260. Add proper wheels with a flipflop hub, and I'll be around $360. Then I still need to do something about the paint and decals on this thing, and I still won't have the Reynolds steel I could've gotten with a Kilo TT, but I will have brakes and some nice looking lugging.

I've probably even forgotten something. I did have nearly all the tools I needed before I started.

All things considered, I'd do it this way again. YMMV