For years I've been trying to get the guy from Boston Pinball to graph multiple machine sale prices over time. Just selecting TAF is not a good representative. He should take 2-3 machines from every era and show their sales, as well as group machines by A-list and B-list and we'd get a much better idea of trends.
For example, here's the BP graph:
There's not enough data to claim that overall pinball values are going up. There is no distinction between EMs, SSs or DMDs. It just shows average sale price is going up, but that's more likely the result of higher-value machines being sold more often. Overall the graph doesn't seem to indicate any dramatic increase in prices. There's not enough data there, but from what we can glean, overal, there is no "bubble" (a real "bubble" would show both prices and number of auctions going up) - there's more likely a bubble in a tiny part of the market representing certain A-list games. But that could simply be the result of high demand for certain games that are not coming up for sale as often.
He only has one example using TAF:
Another problem with the above graph is that it only shows price, not quantity of auctions as is in the first graph, so it's difficult to tell if the prices are really going up or down overall, or simply as a result of limited supply.
TAF is also an odd machine to use as a benchmark. It is popular, but it's also the highest-production game ever. In all likelihood, it should go up in price at a much lower rate than the rest of the average market with lower production numbers.