From what I've seen, I presume that Macs have a roughly V-shaped curve when value is plotted over time. Expensive when they first come out, the value drops over 10-20 years until they're considered nothing more than obsolete and worthless, and then from that point on the value rises as they get "vintage" status and become increasingly rare as the ones that weren't just thrown away when they were worthless are bought up by collectors.
I think the 5xx series in particular has a steeply increasing value because so many of them have literally gotten smashed to bits. If you want one, get it at the going price now, or you'll be paying over $1,000 in a few years.