All I know these days in regards to pinball pricing is that buying a pin is far better than putting the same amount of money in a bank hoping to earn interest. I get to play a game I love and then when the time comes that I have a desire to sell it or life needs dictate so, I almost never lose money. Sure, some games go through "Cold Spells" where it feels like the value has gone down, but what other collectible out there seems to just keep going up in value? Classic cars seem to fit the mold as cited a few posts back.
In my case, buying games "relatively cheap", cleaning them up and playing them for a year or so before moving them on allows me to keep fueling my hobby. Have I bought some duds - sure. Games I paid too much for or games that were basket cases that ended up costing too much in the end - sure.
But overall, it does seem like the value keeps going up over time.
Just my 2 cents. (Next week that will be worth 3 cents)