If you fill out your profile so that we know where you are, we might be able to help you source parts a little more easily.
I got MOST of my tubing from a local scrap yard, and actually had a fair amount left over for some other projects. If you are able to get closer to the exact amount that you need, you'll save some money there.
The plans do offer a lot of advice on building and sourcing IIRC.
My motor I got used on Craigslist for about $80. Depending on where you live, this might be the harder piece to get cheap, but it's not impossible. Instead of buying more expensive knobs, I was able to either fabricate my own or use parts that I scavenged from other tools. Certain other parts like wheels I could have saved some on too, but I went the expensive route and get premium wheels. There have since been a few lower cost options made available from when I built mine.
If you have a welder, you can also tack weld certain peices and save a little on nuts and bolts as well.
It may take a little work and some patience, but it really isn't impossible to build a rather substantial 2x72 for $250-$300.
Take a look at this page:
http://www.mickleyknives.com/html/no_weld_grinder.html
It will give you a basic parts/cost break down, and you will be able to more closely calculate what you need to spend. He's pretty frugal with some of the parts prices, but you can actually find a portion of it even cheaper than he has it listed if you look hard and carefully.