Patience! Where were you four days ago?
A quick [advanced] search on the auction site shows two backs (a pair), just sold for $50 USD each, w/$8 shipping.
I bought one, not too long ago right here in the APUG classifieds for $55 USD. (Can't remember the shipping cost).
From a completed *Bay Auction-25FEB w/only one bidder: Item # 200900867284
*Note: To be specific, these were the 'Polaroid' 550 backs, not the 'Fuji' PA-145 backs. At this particular, point-and-time,
(regarding instant pack film photography equipment), the difference between the two backs is basically moot.
They both get the job done!
not entirely, since only one form of instant peel apart is made