People advertising unrealistic high prices annoys me! It all leads to accelerated price inflation. People get more and more used to seeing and accepting high prices. Sellers who don't know much about HP machines see the high prices and sell at similar levels. Eventually, the base line prices rise because sadly, there are always people who don't do their research properly or simply have too much money to care or can't control themselves in auctions.
I've watched this happen recently with HP85s and people selling junk versions for nearly $300!!!
Anyway, HPs are never going to be a serious investment.
Let's take a 15C at 1983 prices - list price was $120.
25 years in a saving account would yield about $400.
A mint 15C in box with docs would probably reach $400 on ebay today but I'd rather have the money in a safe place like a saving account rather than take a chance on ebay.
If you use official RPI figures (UK) since 1983, the retail price of the 15C today would be $250.
Mark