They often have offers on and the codes are free for the first year (once the product is fully protected), then I think it's about 4p a code after that - if you print them yourself. I'm not sure how much it is if you order them.

The inventory you already have at Amazon will be fine as it only applies to any new stock that you send in. Your stock doesn't become protected as soon as you enrol an ASIN. Usually you enrol the ASIN, send in at least 20 items with the transparency codes on and then after 30 days it becomes protected. After this any new inventory sent in without the codes will be rejected.