The Polaroid 545-545i backs were intended for use with the now, long discontinued, single sheet, sleeved, instant film from Polaroid. Fuji does not make single sheet, sleeved, instant film. Never did IIRC.
Fuji "pack film" does not work in the "single sheet," 545-545i backs. Never did.
Fuji still does make one "pack film," FP100c45, color 4X5 film, 10 sheets a pack, that will work in either the Polaroid 550 backs, or the Fuji PA-45 backs. These backs sell at a used price, well north of $150/USD, and new Fuji PA-45's sell for about $225/USD.
This is a huge expense for shooting a 10 image pack of instant film that sells for about $26-32/USD. And, as of just recently, it does appear that Fuji [has] discontinued production of B&W, instant pack film, in 4X5 size, though there still exists some remaining stock of the 4X5, B&W pack film, at some mail order retailers.
If the B&W, FP100b45 and FP3000b45, has been discontinued in 4X5, how much longer will the color, FP100c45, be produced in 4X5 instant pack film?
As others have mentioned, users have met with limited results when trying to use the remaining Fuji Quickloads in 545-545i backs. In reality, there aren't any/many more Kodak Readyloads out there, as these were discontinued some time ago.
So, to answer the OP's original question; How much are 545-545i backs worth?
Answer; Nothing!
And, they are completely and totally useless for instant photography today.
I suppose one could try using the rollers to make a pasta press.