That is weird, I was only charged about £40 a few years ago to get a new "small plastic" key coded and the missing key removed.
I think that the problem with trying to recode a used fob is, the transponder chip(pill) is only meant to get coded once, so a used fob can not get its chip recoded, when you remove a missing key all you are doing is removing all the coded keys paired to that car, then adding back in only the ones that still exist, also a new key or virgin key, or a key with a new transponder chip will get given an unused code then added to that car's range of keys.
Edit:- the way I see it, if you hand over a bunch of keys some original and some secondhand via ebay to a dealer and ask them to sort it out, then if they cannot code/recode some of these ebay keys, they will not have completed the task you asked them to carry out and still need to be paid, so why would they want that grief just from being willing try to code/recode some ebay sourced keys? I wouldn't take that on.