The SE/30 has been in demand since the day it was discontinued.
Look at materials from very late 1991 into 1992. The SE/30 was a beloved machine when new and was even described in one book as a "perennial favorite". This was in 1994. (Source: "Upgrading and Repairing Macs", published by QUE). Prices remained relatively high until the middle of the PPC era, around 1996-1997, when they started to bottom out--although people would still buy an SE/30 if it showed up at a surplus sale, later on eBay, etc. Rarely did they go for a buck or two like the other Macs. They could be had for double digits in dollars until the past few years, when the "new normal" became $100+.
Regarding Mac prices: in any hobby, prices can and will fluctuate; generally, they will trend upward. For some hobbies, this follows a period of great availability and low prices. Somewhere, there's a 2004 Oldsmobile Alero on a Craigslist ad for about $1000. It's an old compact car, likely destined for its third or fourth owner, and odds are it will wind up with a high school student for the fall. However, in about 30 years, once reality has set in that Oldsmobile has been gone for a long time and the 2004 Alero was their final model, that car will be worth much more than $1000, even if it needs some work done to it. Try buying a 1966 Toronado. It won't be cheap. True, the Toronado was a superior model to the Alero in its time, but eventually, even the low end vehicles go up in price.
Macs are the same way. The 1966 Toronado was the SE/30 of the Olds lineup. Smaller, but full of technology. It's always been in demand, save for maybe when these cars were 10 years old or so. Classics are like the Starfires, Cutlass Calaises, Aleros, Achievas, etc...they're out of production but increasingly in demand, especially since many of the mass-produced models are gone. They have gone up in value, especially for working or restored models. Tack on modern conveniences and the prices can go through the roof.
Events can also trigger a change in price. When Steve Jobs died, old Macs got even more expensive. Everyone wanted to remember his legacy, even though most of our compacts were released after his departure from the company (and before he came back).
When I sell computers, I sell them at values I believe are fair for the condition and the state of the market. The prices I have listed in my sale thread at the moment are not the same prices I would have listed two years ago. Two years from now, the prices may be even different.
