$40 here. $40 there. A year later, you have $200 in your joysticks.
I look at it as money spent on entertainment. If I weren't working on the cab, I'd be spending the money on some other way to entertain myself.
The only cheap route in this hobby is to find a fairly complete machine for <$100, do minimal work to it, toss in a PC found curbside, & sell the original boards to recoup some money.
....but it would probably be limited to 2D games in MAME and definitely wouldn't be
Overambitious.