A core 2 duo machine can run most vpinball stuff just fine.  Those are around 50 bucks.  Add maybe another 50 for a decent video card.  Your backglass and dmd screens should be virtually free... most of us have smallish monitors lying around that would do the job.  So you've got to buy/find a 46 inch tv, get about $200 in wood, some pinball legs, a handful of buttons and some paint.  Table glass and a coin door would be nice, but that might blow the budget.