I just try to keep the expenditures so far apart that the impact of each expense is very little.
I have noticed a tendency for diminishing returns in this hobby, though. I built my cabinet originally for $300, half of which was in the monitor. Since then I have put another $700-$800 or so into it, but although I've increased the cost by 200% I'm certainly not getting 200% more out of it.
My downfall was the lightguns. Once Maximum Force and Area 51 were emulated, I had to have the lightguns. That meant I had to upgrade from DOS to Windows. That necessitated a change in motherboard and CPU, and when that motherboard didn't give me the performance I needed, yet another mootherboard. The CHD's needed a bigger hard drive. Then, of course, if I have Windows, now I have to have Daphne and Visual Pinball. But the USB guns didn't like the serial trackball, so I needed a new trackball. The new trackball didn't fit the old space, so now I needed a new control panel. That trackball wore out quickly, so I got a Happ trackball, which required yet another new control panel. And then I had to try 3darcade, which required another hard drive upgrade to hold all the models and a memory upgrade for performance. And I still have a pair of Pole Position wheels in the garage and a Pole Position shifter, and I'm wondering if it's time to start the madness again....
And the irony of it all is that I hardly ever play Maximum Force or Area 51 now. And I could probably sell the guns if I hadn't modified them to be removable and have the guts of both guns on a single board.
--Chris