The Classic II is a low budget machine and the SE/30 is a workstation. They're not competitors in the slightest and shouldn't be compared as if they were.
I would be surprised if the machines are remotely similar in price these days. The Classic II isn't a cost reduced version of an older model, it has also been crippled.
For a lot of people, that's not going to matter. People seem to have the notion that old computers were only good for word processing and games. That is, more-or-less, what the Classic II was designed for. That is not what the SE/30 was designed for.
The reality is that machines like the SE/30 were targeted at organizations and professionals with deep pockets. Throw in a display adapter and a two page display, and it was used for desktop publishing. Throw in a network card, and it was used as a server. If the standard System Software wasn't good enough, there was even a option to run A/UX. Even something as simple as RAM demonstrates how fundamentally different these machines were. The Classic II was capped at 10 MB of RAM. The SE/30 was certified for 32 MB of RAM, but the hardware design accommodated 128 MB of RAM.
A Classic II can run almost any software that an SE/30 can, a bit more slowly and perhaps with an eye on the available memory. It would also be out of character for that machine. While basement tinkerers are not going to care about something being in character, collectors are going to care and collectors have a huge impact upon prices.
While the top priced SE/30 today exceed the top priced Classic II, for the unrestored machines there is a lot of variance. You can find Classic II for sale on eBay for close to $400, but at times also find SE/30 for under $200. A lot depends on condition and whether the seller is a member of the classic macs cartel. There is a lot of screwing going on in the exchange of old macs. In the early 2000s, Classic II were consistently more expensive than SE/30.
There’s also a nostalgia factor that comes in waves - many people who want to relive the glory days want exactly what they had when young - others want the hot rod they always dreamed of.
