I know they joke about bubbles but honestly, you can only pay so much for these beloved machines. I love this hobby, playing, fixing, collecting, etc... but at the end of the day there is only so much one will pay for those screws, wires, and pieces of particle board with colorful inserts and paint. Many feel your frustration, but think of it as an investment and ride the wave man. Every bubble bursts, even if people say they wont or don't, and pinball prices will too some day. Real Estate, Gold & Silver, Currencies, Markets, College Debt now at $1 trillion bubble, etc. they all fluctuate and eventually burst.
I'd put my $10k in pins over a mutual fund or 401k any day in todays market, but without getting too far off topic, the value of the dollar is declining which means more of your dollars are required to buy the machines. When the kids of the 70's, 80's, 90's, get older and priorities/life styles change, when they go to sell, you may find yourself a deal... but as far as right now in today's market and the lack of the dollars purchasing power, be prepared to pay $4.5k for a late 70's Playboy...
Just my 2 cents.