Used bike from the local co-op for $40. I actually did the volunteer program so I got the bike for volunteering some hours, but if I were to pay for it they valued it at $40.

$bike - seatpost
$bike - saddle
$bike - bb
$bike - crank
$bike - pedals
$5 - toe straps
$75 - wheels
$bike - chain
$30 - freewheel/cog(s)
$bike - stem
$bike - headset
$bike - bars
$5 - tape
$30 - tires
$10 - tubes

Overall I spent about $175 after shipping for all of it, plus some tire levers and presta/schrader adapters.

I built it all at the co-op so I didnt need to worry about buying any tools.

You can get away with doing it pretty cheap or you can replace everything and spend alot at which point you probably should just go buy a new one.