The current (and growing) value of S30s is rather a unique formula of factors that you don't see in many other markets.
Being that they were relatively affordable when new, they are very relatable cars. If you have never owned one, chances are you knew someone who did. We see this frequently in the BaT chat when "non" Z enthusiasts happen to find one up for sale and can relate a story about one that was the same color, year, model etc. You don't see that kind of personal connection in the Porsche 356 world for instance.
Until recently, they were "cheap" cars, and that's reflected in the grade of restorations you see in Zs that were done 15-20 years ago. I think Maaco could have used a S30 as their marketing platform. Seems like every Z I've came across as a project for sale had a horribly shot paint job, the whole "lipstick on a pig" appearance. Even though it was mass produced, a genuinely unrestored S30 these days is quite rare and why the lower mileage examples fetch a pretty penny.
The part availability market probably has a contributing factor to the value of S30s as well. You can't simply swing by a Nissan dealer and grab a new dash, or pick up a catalog and order all new glass for a S30. I use to envy Mopar folks because they could basically build up an entire car with new parts, starting with nothing more than a VIN plate and title. Now, we see S30 values knocking the staple muscle cars out of the water. I feel the fact you have to work hard or pay $$$ to find those hot items for S30s (think D-hubcaps, pointy antenna, bumper overriders, correct hose clamps, correct exhaust system) that when you do have all the pieces to the puzzle, restore a car properly, and get it in front of an audience that can relate to the car, people start to do funny things with their checking accounts.