--I needed to clarify this--
New Stern's RETAIL PRICES have gone up 40-60% over the last 7 years as well, observing the pro/prem/LE business model. (Not "cost")
I don't know where the "breaking point" is for buying an older machine vs. a new Stern.
I feel most buyers will always go with their priorities. If you are looking for reliability at home, or looking to put a game on location, you almost have to go with a Stern pro. If you can work on games yourself, like tinkering, or just want to play the game you did 30 years ago, you might prefer older machines.
The money factor is almost not an argument these days. There are pins from the 80s that are $600-$6000 and pins from the 90's-2010's that are $1500-$12,000. The higher priced ones are that way because of availability. With vintage pins, the lower priced ones are that way because they are either trashed, broken, or just not very good games. $500-$1000 pins from the 70s-80s are that price for a reason.
As a generalization, the newer pins are going to be much higher priced for other reasons. The BIGGEST reason would be the game's FEATURES and what is there for the player to actually experience in a few games... the more the better. It's actually harder to make a "timeless" game hold up in the current sea of competing modern pinball features.
You don't really need to compare a game from 1982ish with a game from 2012ish. The OLDER games are priced the way they are because of supply and demand.
The newer games are priced the way they are because of MANUFACTURING with the prices being set at 5-8ish+. They are not really PRICED by features. It seems that only the DEMAND for individual modern pinball machines is swayed by features or to a lesser extent, "completeness". That is usually not the case with vintage/older machines other than maybe a few one off machines like Safecracker or Banzai Run.
So for me, 1-2K seems like a comfortable spot to be for a vintage machine that may need some fixing up to make great.
It seems the older games are currently about a (three to 1) or (four to 1) ratio to a newer Stern. THAT ratio actually hasn't changed much over the last 15 years.
One closing observation. Since the year 2000, the price of a used, nice condition TAF has always been about the same cost as a newly manufactured Stern Pro-Stern Premium. (Observable at the Boston pinball selling archive.) This is more important than you think because even if you hate TAF, it sets the standard for what is considered a timeless, classic, and collectible pinball machine.