Quoted from ForceFlow:If you really want to scratch build a game, you would probably be better off designing and building your own custom game.
I'm building a Scared Stiff from parts. Just picked up the coffin weldment, spider assembly and a transformer this week. I'm doing it partially to understand some of the design choices made. And I'm reverse engineering a lot(driving the dmd from a Raspberry Pi with pinmame, using my own controller boards for the real time components, adapting parts from other games or making custom parts to enhance it, etc). It's more a labor of love and learning, but I've got the vast majority of the machine sitting in my garage, and I maybe have $600 in it, counting the set of decals and the playfield I picked up.
So, if it is something that revs your motor, go for it. Worst case scenario, you learn some stuff and spend some cash.
EDIT: I should note that some good fortune landed me a free cabinet and some wheeling and dealing scored some parts for well under what they would normally go for(the playfield, for example, was a very nice PF that was only swapped out because the owner was going for a really high end resto). And I'm ok with functional used parts that I can later swap out, so my ramps were cheap, etc. If I were building this with the mindset of a factory perfect game, the price would be easily thousands more.