[jrockway]

You can build a cheap fixed gear bike to be cheaper than a cheap derailer bike. I haven't priced things out, but I bet an OEM can do it for less than $150. The only parts you need are a frame, fork, seat post, saddle, stem, handlebars, one brake lever, one brake caliper, one short brake cable and housing, bottom bracket, crankset, front wheel (hub, rim, spokes), rear wheel (hub, rim, spokes, cog, lockring), and chain.

As soon as you add a derailer setup, you need an extra chainring, rear brakes, shift cables + housing, the derailer itself, 10 cogs instead of 1, and expensive shift levers. (Even Shimano's low-end 9-speed shift levers are in the $150 range. The good ones are $700!)