I could advise in terms of 21 backs (LXW, DSD, Power droid, R5, Hammerhead, Snagg and Wally)that I wouldn't expect to pay over £500 for a nice 80 grade for any of them apart from for Greedo and of course Boba. Most around £350 -£400.
Can't advise re 1st 12 on 21 backs.
For Palitoy, double...