They sure add up quickly, don't they?
My first version was so cheap because the computer was free except the power supply; the cab I got on eBay for $61, and its control panel already had 2 Happ Ultimates, 12 buttons, two start buttons, and the cab had the glass, car speakers, and a complete coin door sans mechs; and for the interface, I hacked a free keyboard, which had an embedded trackball that I was able to make work in the cab. My biggest expense was a monitor for $150.
Since then I bought a new motherboard and CPU ($150), side art ($70), trackball ($10), lightgun ($40), better joysticks ($16), Golden Tee and House Of The Dead 2 ($20), more buttons, more screws, etc, etc...
--Chris