Ironically the one thing the customer hasn't complained about was the cost. Our beef is with the Fire Officers guidelines to the customer.
If only it were always this simple.
Work on 2-3p per tag Mike. 
I am not sure about the actual cost of an individual tamper tag but if it is challenged when included in the service the answer would easily be 'Okay you don't want the tags changed. It will reduce the overall bill by ?0.20!'
3-4 years ago we quoted an all in Basic Service price to service a Central London Hospital and guaranteeing the price for 5 years. Had been there a couple of times during the negotiations and everything looked fairly straightforward. Knew it was an ex Chubb contract and didn't really give it much thought other than parts are easy to get hold of, hoses and bases all compatible with TG. Something like 3000 extinguishers. Won the job.
Customer told us they always carried spares and that facilities would swop extinguishers when they found them tampered with to reduce call out costs. Customer said service spare and tampered with first.
First day of servicing and the customer shows us to a store room. 60 Chubb FX extinguishers with pins and OK indicators missing and 50 brand new Chubb FX range extinguishers still in their boxes. First time I had seen the FX range.
Pins ?2 each, OK indicators ?1 each. Had only calculated for the old style Chubb frangible pin and OK indicator.
If only they were 2-3p each and ?230 done in one fell swoop plus ?50 a day parking and Congestion Charge.
