On jobs I have done in the past, I would figure-out how to tie mine into the existing system before giving a price and include that, even if not required. A quick talk with any reasonable homeowner would convince them that their family's safety would be worth the extra $$$! EASY SELL! Even if the existing system is a discontinued model, factor-in changing ALL of them to new! I have, in many houses, added smokes in all of the upstairs bedrooms tied-into the existing hallway smoke. 3 pop-ins, some 14/3 romex and 3 new SDs is a great improvement to any house!