Have you thought about converting an older bike into a SS/FG? That's you're most economical option. I bought a complete 1970s Centurion that I converted to fix geared and it's great. Most old roadbikes with horizontal dropouts can be converted quite easily.

Check out Sheldon Brown's article on Fixed Gear Conversions if you haven't already: http://sheldonbrown.com/fixed-conversion.html

If you're wanting a new frame/fork you might check out the Eighthinch Scrambler frameset. I've seen them for $150 and so far I've been happy with all of my Eighthinch products.