generally, it's pretty difficult to judge what the decay is. an unscrupulous person could swap kits. also, the type of kit is important as is the skill level of the one using it.
I always felt it was the responsibility of medic to be efficient at what they do. so, they need to choose the right kit for the right task. so, a well prepared medic will carry several. the decay should be their responsibility.
if I like the job they do, then I will pay them what I think it is worth. so, it on a tip basis. I expect the same when I work as medic.
yes, you may get a client who doesn't tip or tips poorly. you just remember them for next time and don't work with them. as a customer, you may get a medic who has no clue what they're doing (disrupting the herds, dying all the time, causing you to die etc...). again, pay what you think the service is worth and don't use them in the future.
being a medic is more that just clicking your kit on them. a really good medic also functions as a companion (should they wish someone to talk with) and a guide (knowing the area, herd sizes, approaches, etc). if the customer is a miner, the medic can also function a secondary guard as well.