I was actually in the same position as the OP.
Only bike I had was an old MTB from childhood.
I decided to build my own single speed. From the moment I bought a frame ($100 "Nekkid" track frame from Nashbar) I knew it wouldnt be cheaper than buying a SS/Fixie from a place like bikesdirect lol.
I still went through with it though, cause I'm the type of person that likes DIY projects.
Started last... August, and took a pause since I ran out of funds and it was winter anyway.
Bike is pretty much finished now. How much did it cost..? I'm actually not sure, I'll go and use carleton's post to calculate.
Maybe my experience doing this will help you decide.
Will edit in a bit with price info.
Frame: $100 - Nashbar Nekkid Frame
Fork: $80 - Performance Bike Forte Carbon Fork
Stem: $25 - PB Forte Stem
Headset: $50 - Cane Creek 40
Bars: $15 - Forte ATX bars
Grips: $20 - Crank Brothers Cobalt Grips
Wheels: $150 - Velocity Deep-V w/ Halo Coating
Rim tape: $10
Cranks AND BB: $47 - Vuelta Pista 170mm 144BCD + Sugino CBB-103 (won on ebay)
Freewheel: $55 - White Industries ENO 19t (won on ebay)
Chain: $10 - KMC Z-410 (Nickel Plated)
Tires: 2x$25 - Panaracer Tourguard 700x28
Tubes: 2x$4 - Nashbar tubes
Seat post: $13 - Kalloy 27.2x350mm
Seat post clamp: $7
Saddle: $26 - Charge Spoon
Pedals: $15 - Wellgo Pedals
N/A to you, but I built SS:
Brakes: $50 - Tektro R580 (pair)
Brake Lever + Cables: $30 - Shimano BL-R550
TOTAL: $741
That is not counting $$$ i paid for tools, lube, etc.
Does include shipping for the most part though (I think).
I didnt try super hard to go on craigslist and find deals.
But keep in mind, prices in normal retailers will probably be going up since its approaching spring now.