As many have said, there are often tangible differences between even keyboards that use the same switch.
Cherry uses nice keys, but have lots of flex due to being PCB mounted.
Filco apparently had some controller issues, that EK shielded his buyers from, and uses pretty crappy keys, but is otherwise very well built.
The Model M is built like a tank, but is generally older, and not consistent between models due to spring fatigue. This can be avoided by just buying from Unicomp, but the looks are...lacking. I love mine, but it's definitely not for everyone. Layout customizability with a bit of work and some software is trivial.
Ducky makes an inexpensive board, but has reliability issues. And crappy keys.
Noppoo doesn't work on a Mac.
The RealForce is amazingly well built, and amazingly expensive. They do feel amazing though. Not worth the money to many people. A bargain at twice the price in the minds of many others.
The HHKB is not quite as well built as the RealForce, but in the same league price wise. However, it's compact form and UNIX layout is a huge draw factor for most of its owners -- myself included. It's only about the price of 5-6 new Sun type 6 keyboards, which is about the number that I wore out in the 2 years prior to purchasing it. It still feels brand new.
The Deck is a modder's delight, in that it can be opened without voiding the warranty. They are also the only readily available keyboard with Cherry MX clears.
With the exception of the Topre switch keyboards, most of them are in roughly the same price range. You pay more for some based on quality, some based on name, some based on size, some based on features.
It really all comes down to what you want out of your keyboard. I use Model Ms and a HHKB, all of which have the UNIX layout. That makes me crazy by some folks' standard, completely rational by others' standard.
I'll be honest, I am considering a second HHKB purchase. Yes it's expensive, but for the way I use my keyboard, it's worth the price. I am on it 12-14 hours a day, writing code and my dissertation, plus random chatting and commenting on these boards. At this point, that puts it at roughly $0.40/hour of use, based on my purchase date and price.
For the record, I think buying a Cherry switch keyboard is wasting money, but that's because I find it uncomfortable to type on, so for me, it is a waste of money. And just like the RealForce line, they all have the wrong layout. For you, it might be amazing. You might hate BS keyboards, while your neighbor may love them.
With something like a keyboard, especially if you're a heavy user, many of us are willing to spend as much as necessary to have one that is comfortable and long lasting. I know I am.
Edited to fix plate/PCB conflation. Thought one, wrote the other. Oops.