With the current electricity prices in Germany you'd be paying around 30 €/month for a 100 W always-on server. And enterprise servers idle at around 200-250 W, so there's that.
Compare that to a RPi that idles at ~5 W.
Compare that to a RPi that idles at ~5 W.
They seem to be using the 8GB RP4, which is currently around 100€. Also it comes with a 2 TB SSD for storage which is probably another 150-200€. A 2 port SATA backplane/adapter is probably 20-50€, Power supply, fans and wifi/bluetooth antenna is maybe an additional 30-50€. Even the cheapest case will set you back another 10-20€. So all told the markup isn't as bad as you're making out. Still an open question if it's worth it or not though.
FWIW, I've got a PiBox on order (w/o the drives), and I think the price is VERY reasonable. (No relationship other than waiting customer.) The PiBox "mainboard" that hosts the CM4 has much better I/O capabilities than a garden variety RPi 4. I'm actually going to be using mine for a small, light NAS and application server, and I'm confident it will work well.
They also sell the box without the Pi, SSDs, antenna and fan for 150€, it is up to you to decide if the price is justified, I think it is a bit on the expensive side but not unreasonably so. But if you intend to DIY the same configuration they are selling for 540€, you won't get that much cheaper.
Where are you getting these measurements from? A "consumer-grade PC" idles at a few watts[1]. "enterprise servers" might idle a bit higher but nowhere near 200-250W.
[1] https://www.notebookcheck.net/Lenovo-ThinkCentre-M90n-Nano-D...
I made them myself with a kill-a-watt. The consumer grade PC's idle at ~60W, meanwhile the Dell PowerEdge R710 I have here with me idles at ~250 W.
However, the easiest way to have low idle power is to use a NUC-like computer.
Many models consume between 5 W and 10 W during light load at the wall plug, while those with the highest power consumption use between 10 W and 15 W at the wall plug, during light load.
The fastest SBCs with ARM cores that can be found at a reasonable price are comparable in speed with Intel Jasper Lake, but Raspberry Pi uses obsolete Cortex-A72 cores, which are 2 to 3 times slower.
Even a 7-year old NUC-like computer from Intel/Gigabyte/Zotac etc., with a "Core" CPU, e.g. a Skylake U, is much faster than anything with Jasper Lake or with Cortex-A76 or older ARM cores.
While a new NUC-like computer is more expensive, an old one might be as cheap as a Raspberry Pi, while being much faster.
During the last 20 years, I had a permanently active 24x7 server, which hosts a large number of services, e.g. firewall, router, NTP server, DNS server, DNS proxy and cache, e-mail server, Web server, Web proxy and cache, file server, etc.
The first server which did all that used the fastest Pentium 4 and it had an average power consumption of over 200 W. The current hardware of the server is the 5th version since the beginning. At each upgrade, the average power consumption has been cut into half. The current server consumes around 10 W and it uses an Intel NUC with a Coffee Lake U CPU, together with 4 USB to Ethernet adapters, to provide 5 Ethernet ports.
I have experimented with various ARM-based SBCs, but when old NUCs are available, repurposing them is a higher-performance solution.
Current Dells pull that much without even an OS installed, just sitting idle at a boot screen.