That £75-£100 cost per letter is probably a figure pulled out of thin air by the CBI, probably by one of their mates who owns a bank, and thus uses bank costings for working out such things. In other words, an out and out lie.
I suspect that nearly all of their mess-ups are due to their stupid habit of saying that each source of income must be owned by a different office, rather than have one office in charge of each persons reckoning. As a second best, swallowing the idea that offices should be able to talk to each other would be a step forward. They might have done this (anything is possible), and this could have thrown all this up.
As it is, the work is done by the people at the bottom of the pile. Their qualifications, while claimed to be equivalent to old fashioned "O" levels, seem to have reading and writing with spelling, punctuation and general grammar as optional extras, and basic arithmetic as a lucky guess at a multiple choice sheet.
The ones making the rules and setting policies and the means of carrying them out are usually the breed who are good at talking themselves into a job that they have no talent for whatsoever, usually in front of a board of similarly equipped people who are looking to the future, and don't want any competition.