cheaper to buy a brand new ready made bike from a store? i disagree
by scanning ebay and buying the cheapest parts you can find and finding an old road bike in a dump/rubbish pile out the front of someones house you can build up an 'acceptable' fixed gear bike for under $200

i'm not saying for this money you'll have a fabulous quality bike but it's something you'll have built for yourself, you'll know how it works and you'll know what you like and dislike when it comes to upgrading later... also you get some experience on cheap materials so if you break something in the beginning it wont be so bad money wise