1st - conversion, was around $550 (existing computer and monitor, 2 player, trackball).
2nd - scratch built bartop, project cost around $400 (existing laptop and monitor, 1 player + trackball) and a LOT of time.
3rd - scratch built driving cab at around $1,200 (most major components purchased used).
4th - current conversion project, will come in somewhere around $700 (existing computer, purchased LCD, spinner, lots of lighting effects, etc.)
For what it's worth.