I have both an ETRS and ETR-Si, neither came with a WLF. If you ever find a deal on them, I'll take one
My understanding for why they are so expensive is that there are very few left. The ETR series was hugely popular with the wedding photograhper crowd, and they primarily used the AE prisms. So, they'd buy the prism, and then throw away the WLF thinking they'd never use it. I purchased my ETRS from a retired wedding photographer and when I asked if it had the WLF he laughed at me. It wasn't a mean laugh, more of a "why would anyone want that?"
As younger people pick up Bronicas, more and more of us want to do things other than (just) shoot portraits, and the WLF is once again popular, so supply and demand says they are expensive.
Seriously though, if anyone has one they want to get ride of for $40 let me know