I saw an instructable for a TRX under $25 that suggested It could be made of paracord, I took that as challenge and made seven of them, for Christmas gifts. Each was under $9. So, to start you'll need to gather three materials: Paracord, Handles, and Carabiners.

Step 1: Knots of Fun

The next thing you need to acquire is knowledge and skill in knotting. You need to learn and execute four knots, the lark,s head, the bowline, the constrictor knot, and the rolling hitch. The constrictor knot is also known as the constrictor hitch on one site so be aware. You need not become facile in their application, but for your own safety you should tie them tightly and efficiently.There are several sites with pictures or animation, of methods to use these knots and hitches.

Step 2: Lark's Head or Cow Hitch

http://www.ehow.com/how_13480_make-larks-head.html
Use this to make the connection between a 6'0" piece of Paracord and the middle top caribiner.

Step 3: Constrictor Knot

http://www.realknots.com/knots/constric.htm
This knot gets tighter, as the name suggests. Use this knot to tie one carabiner to each end of the 6' para cord. This completes the top center piece.

Step 4: Rolling Hitch

http://en.wikipedia.org/wiki/Rolling_hitch
Cut two pieces of para cord such that they are long enough when doubled they will fit the hypotenuse ' c' in a triangle a,b,c, where a=9', b=your height (to the closest foot) its own carabiner at the end of the top center piece. Create a loop by tying a rolling hitch.

Step 5: Bowline

http://meritbadge.org/wiki/index.php/Image:Bowline-small.gif
tie the handle on with a bowline.

Step 6: Resources

Many books of knots exist out there, the standard, by acclimation, is 'The Ashley book of Knots.' But at $80, retail it's a bit out of range for some of us. I've put some links to websites for the knots, I've used them all.
good luck