As I said, I just found mine (135w for ETRSi), but there are still two left on eBay. One for $299 and one for $329. That seems to be a going rate.
If you look at eBay and do a search for 135 W at least three times a day.... once a day is better.
Since I buy/sell film cameras on eBay fifteen years now, I'm there every day. I usually find the hard to find stuff, more than once very often. I purchased three Bronica film backs just this week, for my own use. I may sell the SQ one, once I see how fiddly the ETRSi one really is.
I have not found anyone say anything good about the back for the ETRSi, but then I have never found anyone who has used the one for the SQ.