used or new: fuji , scott , and giant are the usual suspects that offer best $/g values.
if you build your own, just keep in mind:
*sram is the weight champion of the big 3
*sram red 10sp (used, obv) if you really want to do price effective weight weenie
*easy weight savings are in wheels, tires, saddle,
start off with big 3:
(1)frame/fork = 900g/350g @ 600$ shipped
(2)groupset = 2300g (ultegra 11sp) @ 600 shipped
(3)wheelset = 1500g @ 500 shipped [see example list
http://road.cc/content/buyers-guide/...r-choices-new]
prices and weight are approximate as im in a different geographic market and currency than you (so theyre both overestimated, actually), but basically its a good foundation for a 6.8kg build. mine came in at that weight right on the button.
the real cost will be all the special tools like like bottom bracket tools and cassette wrenches and cable cutters.... but its a good 'investment' anyway and a fun exercise to do.