I think even though there are more collectors, I think the buying pool has actually gone down with the economy a bit. The problem is, those who can still buy, are willing to pay more, which is driving up cost.
I talked to a lot of people about it. There are much fewer listings than there was before, because people are worried that they won't be able to buy it back 2-3 years from now if they want to. As a result, there are less machines on the market, and because people aren't listing, they're not adding to their "pin funds." It's just people who can add a machine to their collection because they have enough cash already to do so that are buying. Lower supply mixed with people willing to pay more will always lead to higher prices.
There's no such thing as a bubble that doesn't shrink, however. In pinball, the inflation and shrinking might seem lopsided toward the inflation side, but as with all things, there will be a correction in pricing if more and more people get squeezed out of the buying pool. I'm out. To get my Tron I had to sell one of my all time top 3 games. I don't have the cash on hand like I used to. The economy dictated that.
There are a lot of older collectors who are deeper into their careers, or retired, who aren't raising kids anymore and they have plenty of disposable income to sink into pins. I think they're the biggest driver in price inflation right now. Eventually their limits will get pushed though, and things will have to reach a stabilization point.
Edit: one more thing. There's very few project machines being sold anymore. There's too much money to be made by fixing them up yourself before selling them. A lot of the great deals disappeared along with the project listings.