I'm in the middle of making mine and so far I've spent around $550 but that doesn't include the tools them selves, butit does include saw blades, saw horses, all the wood and parts to build it, all the controls, 20+ buttons, trackball, 3 joysticks, t-molding, an I-Pac, etc. Buying everything from ponyboy instead of from happcontrols.com saved me $50-$100, so I'd recommend doing that.
I already have an old computer I'm going to use, so all I really have left to buy is a monitor which will probably be around $100 more and probably another $100 on other random stuff (artwork, speakers,etc.)
So when all is said and done I'll have spent around $750 for it.