I've been on both sides of the fence, and it really depends on how important the game is to be part of your collection. If it is a "Holy Grail" pinball, then the price should not be that important. I think that if you have done your homework and are committed to a title, then buy that machine and enjoy the game.
If you are just window shopping and then get "caught up" in the moment (bidding war?), then your DESIRE for the machine is more than you budgeted for. This is demonstrative that you want the game MORE than you initially thought. The point being, it is now in your gamesroom, so you should simply enjoy it.
The only time I suffer disappointment is when the game is not as described, and you need to spend a lot of money on it.