(I mean all of this in a non attitude- thankful I found Pinside and got my first pin kind of way.)
First of all - Pinball is a very reasonable hobby!!!! Especially for how fun it is! Look around for other collectables and see for yourself. Fyi - Boating is on the high side.
You can get the grail of pinbal MM for what $12,000, $20,000 for the cherry/best one around. Compared to the grails in about all adult collectables and hobbies that is nothing! If you can't afford that buy a Pinbot or F14 for $1000-1200 and have a blast of your own. Add more later if you like it and can afford them. It is that simple.
C'mon, man! Pinball owners do not dress, look and have as much money as that Monopoly guy!
If you are an adult and can't afford $1000 for something you really like you either need to budget better or work harder. Bring coffee and bag your lunch and a pinball will magically appear in your basement!
The toughest part of buying a pinball is finding one locally in good shape that you have on your wantlist. You pay not what a seller wants, but what it is worth to you. Though the next guy may value it higher.
You have some really nice people locally and online who are willing to help a total stranger adjust your broken flipper. Unheard of in most collectable markets.
Stern is a big part of the pinball growth with a great run of games, especially the last 5 years.
Nostalgia plays a roll, but the play of the table's rule. In the end people will pay more for a newer pin with more toys on the playfield than an older pin that is great as well, but has less bells and whistles. Newer technology is fun and can/will draw new players.
Stern may actually help keep older pins prices from going through the roof. Example - do I want a Monster Bash or ACDC Premium? A great decision to make, but these newer great pins at similar prices keep the older pins in check.
If there is a bubble, what is the worst that will happen?
Answer- You have fun playing your worthless pinballs in your basement with your few friends who still play and you laugh when someone tries to sell their Medieval Madness on Craigslist for $4000 and then say "that is way to high, what fool would pay near that?!" You then get it for $2500 and he delivers it and throws in a Pinbot to get the deal done. That is soooooo horrible!
All of these things mentioned have a part of making pinball more popular, but still what 1 in 1000 people have one in their homes? It is a growing bubble, but a tiny one.
So buy what you can afford and learn how to wax a playfield and have fun and remember that one of best parts of Pinball is the people.
Cheers!