They go used every once in a while. I have bought and sold them for under $200 over the years. I'm sure you could figure out how to build one. The only thing worth the money is that they have figured out the length of the rods and arms and the geometry to make the thing work.
I had disposable income when I bought mine years ago. I think they are worth the money since it would take a couple weekends to make one and get it working, plus some cash for pads and springs, pins, bolts, abrasives, drill bits, etc. That being said I would probably make one these days since I don't get much loose cash around.
