Well, I am doing basically that, building one from scratch. Here's what I've got into it so far, and what I have yet to do. Mind you I'm going further than one has to go, but I'm not going as far as some have gone.
$20 60cm Frame w/ fork and headset, used.
$25 27" Wheel with Suzue Basic hub (Normally $100)
$50 Seatpost, Seat, Tires, Rebuild for front 27" wheel, tubes
$100 Cranks, 42T Chainring, Bottom Bracket, platform pedals
$20 Shimano 16T freewheel
That's all approxiamate, though fairly accurate. This does not cover a small ($40?) investment in tools that I've made. Overall I would say $300 would cover you, if you had to buy a new wheelset. You can do it cheaper, if you re-use a threaded hub and re-dish and re-space it, and use the current chainrings and stuff like that. But if you were to do it from scratch, not having anything to work with (such as how I'm doing it) then you'll need alot more.
I also didn't cover brakes or brake levers.