Prices seem to rise and fall for no apparent reason. Plastic SLRs from the 1990s don't hold their value as much as the metal-bodied counterparts from the 1960s and 1970s, even though the newer cameras are more advanced.
Certain cameras seem to hold their value.
A Zeiss Ikon Contaflex, Hologon, Contarex, Tenax II, Super Nettel and Nettax (35mm rangefinder) haven't budged much in price over the past six or seven years.
Some of the old classic cameras have increased in value. But some haven't.
The Pentax K1000 has come down in price a bit after rising. They're still decent bargains, and I've bought two in the past couple of years.