I think it's just low supply. They were quite expensive so not that many existed to begin with, and then the last generation of SGI MIPS machines were largely cleared out by companies and academia in the mid-late 2000s. Anything left for sale now is either from hobbyists, surplus sellers hoping to cash in on the few niche markets that still use these, or just really late clearouts.
I bought my Octane on eBay in 2008 (it was previously used at Ford), I think it was around $80 and shipping was more than half of that (an Octane weighs ~80 lbs).
In the late 90s a lot of these machines were being abandoned. Properetary Unix was dying due to Linux, and non-intel CPUs were also getting crushed. Plus there was the dotcom crash meaning defunct companies got rid of gear.
In those days I got an extremely cheap Sun workstation on ebay as an example.
Anyway, I guess supply is low but there are still nerdy people who think an old Unix machine is cool. (I'd take one running Irix just to play with for sure.)
That is more true of old x86 hardware and older micro-computers (Amigas, Ataris etc).
The SGI machines are difficult to get hold of in the UK to begin with plus they are as other said short supply. There is one Octane in the country and it is the machine only (even though the monitor and keyboard are pictured) and it is going for about £800 on ebay.
If it was everything I would have said "that probably the best deal I am going to get" and went for it.
I believe they are used in certain models of CT scanner, MRI or some such expensive medical equipment. That probably drives at least some of the demand.
