The whole process can take half a day but I don't think it's appropriate to charge for all of that. My charge for a straight-forward clean is just for two to three hours of my time; between 120 and 180 GBP plus VAT, assuming that no parts are required, of course.
Okay, so I never fix 'one' key, as a single key going down, is indicative of an excessive accumulation of dust and other airborne contaminants on the key-bed. I therefore, always service the entire keyboard.
Stripping a keyboard, cleaning the keyboard contact PCB, cleaning the keyboard contact PCB carbon contacts and cleaning the carbon nipples inside the rubber bubble strips is extremely time consuming. For a 61-note keyboard, the process can take between two to three hours. Then, the keyboard needs to be dried out and reassembled.
The process is less involved on keyboards that use copper spring contacts but in other ways, they're a lot more fussy, requiring precision alignment, for example.
Honest... it's worth it in the long run. If not done properly, then you're only going to be back in a few months because another key has died!