I'm only at the prototype control panel stage, but my preliminary calculations are as follows:
Buying vs Building - the cheapest option is somewhere in between.
It would be cheapest to get a used cabinet. Possibly in working condition. You should be able to get one for about $200-$300 working if you look in the right places and you're patient. Then all you need is an old PC and monitor, some wood filler, painting supplies.
Compare this to building from scratch with new materials. I paid 250 for control parts, and I'll have to pay $100 for wood. 350. Plus wood filler screws, painting supplies.
If you can keep your used cabinet purchase to 250 or less, you can buy replacment controls, switches, coin mechs etc and still come out cheaper than building from scratch.
The trade off is, if you design your own cabinet and build it from new materials, you can get the exact design you want.