All rares look like they have decreased in price over 180 days. Probably has to do something with bot nuke because a lot of money left the game causing the inflation to go down. I don't know much about the RS economy, but that would be my guess.
I've never bought rares which you can't equip, they just seem pointless if they're not for show. That's just my opinion, and it's also due to the fact that most items over 50m are decreasing in price...