You might want to take a look at how I did mine. I did use the gas pipes at first but had problems with flex. I then tried filling the pipes with cement. That did help but I still had too much flex. I then did away with the pipes and mounted angle iron to the lower bed. The bearing assy that used to ride on the pipes was then mounted on the bottom of the gantry and now ride on the angle iron. On the bottom of the upper table I mounted another piece of angle iron and installed two more bearings to prevent the gantry from tipping over. You can see my machine in action at youtube. [nomedia="http://www.youtube.com/watch?v=aWwokuCNBEo"]YouTube - DIY CNC Router (JGRO) Toolbox Foam Layout pt1[/nomedia]
Take a look around 3:40-4:40
I cant remember what this cost but it's got to be the cheapest option other than JGRO's gas pipe idea.