Not at all difficult to add a camper back curtains to a quality Bimini. I had Harbor Custom Canvas in Long Beach, CA. do that to my first 22. As I recollect I didn't have screens made, just the clear windows. About $1200 at that time, but I had been a long time customer, which may have made some difference. For my Tom Cat the outstanding camper back without screens was over $4500. It had free standing Bimini, with 4 bows of 1" SS. We used Sunbrella Plus and high quality window material. We had a 2' wide section which would zip out for boarding. My current 25 Bimini also has the first 18" which will zip out, (zipper in front and back, with velcro to seal, and make watertight.)
I bought a Camper Canvas from King for my first 25 ($2500)--it was poorly made, and I was not happy. My 2nd 22 had a King Camper canvas, and it was well made and a quality product. It may depend on who does the sewing and what the shop's policy is.
My current 25 was advertised with "full Camper Canvas"--when I got there, it had full screens only. No way to keep the water out. Building on an idea I got from Mark( Marco Flamingo), I made an entire set of windows/enclosure, with 30 MIL. clear Vinyl (least expensive, very flexible), I used ShelterRite, which is an 18 Oz vinyl boat cover material, with a Polyester Scrim weave. Very strong and totally water proof. I attached it with Common Sense Fasteners to the upper part of the zip in screen panels. Along the over taping edges I also used common sense fasteners. So far it seems fine to leave the lower edge free, inside of the snaps of the screens. However it would only take couple of hours to add snaps to the bottom if necessary.
The total cost was less than $500, Including enough fabric to make a cover for my Honda EU 2200i and a large rain fly to go over the entire foredeck, so I can keep the center window and forward hatch open in a heavy rain storm. The cost included a better 1/8" punch, a better commercial grade snap fastener anvil and flare tool, a common sense fastener grommet punch, hard rubber cutting block, and a pair of 8" quality shears. (All of these were about $100, and all that was necessary)- The entire camper "canvas" was glued, including to the windows with HH 66 Vinyl glue. It took over less than half the time it would have taken to sew the camper canvas, and was significantly cheaper. (seeing I already had the screens). Disadvantage, is that the screens have to go up first, then the water proof enclosure goes inside of the screens. Advantage--it really is water proof and I took the waterproof enclosure all the way back to include the splash well.! This same technique could be used with a good Bimini--but probably you would want to re-inforce the edges for the Common Sense Fasteners--and having the "zip out" windows or smiley faces would be difficult, if not impossible. we never used the smiley faces or zip out windows. We would throw the sides up over the top of the Bimini. I made adjustable straps, with snaps which will hold the sides over the top.
With the current setup--I can either strap the curtains over the top, or putting the curtains up or down, only takes a couple of minutes. I will do a tutorial on what I did as a part of the "Thataway" update.