I think it is always wise to focus on what a helmet or anything else that is not necessary for daily comfort is worth to you, regardless what the market price is. This doesn't mean you have to throw money in the dustbin, but that you have to put your own goals and thus avoid disappointments. People buying pickelhaubes only as a form of investment create and support this increasing stream of fakes, not the serious collector with a long term view.
A good example of the inflationary and absurd situation that Chris described is the so-called tulip bulb crisis in 17th century Holland. Around the 1630's, there was a very increasing demand for tulip bulbs, cultivated in Holland, from the French court, and prices went up steadily, until 1 tulip bulb became as much worth as a respectable house or 20 times the year salary of a craftsman. Of course, everyone was trying to cultivate these tulip bulbs, often of very inferior quality, and the offer became bigger than the steady demand. Prices went down and merchants were selling their bulbs at any price. Finally, the market collapsed and numerous merchants went bankrupt. As a result, around 1640 the price of the tulip bulbs returned to the normal supply and demand situation.
Johan