What you as the customer should know and can see from the answers posted herein, that recovering refrigerant into a recover tank and re-using it in the system is a conjecturabley acceptable practice. Using virgin refrigerant in a system is never conjecturable. It's just a conservative method of guaranteeing that the system is internally maintained in close to a perfectly clean state as possible. Some folks will do what's referred to as a 'pump down' where the bulk of the refrigerant in the system is captured in the outdoor unit, from where it can be released at the appropriate time, back into the cooling system. It never, in that case, leaves the system and therefore is as clean going back in as it was coming out. Whether that's optimum clean or not is an unkown. Clean, virgin refrigerant is a known.
So perhaps your tech has quoted you on the worst case situation and you'll be pleasantly surprised if he can lower the costs. Or perhaps, as others have suggested, he's just going to do it 'by the book', a method no one can second guess.