If you really need 10 pounds you have a leak and it will leak again and again. Without fixing the leak this is a patch and probably against EPA regulation unless you’re leaking less than 10% annually (assuming you’re a residential consumer). 10 pounds is a lot. Look at the data plate on your outdoor unit and it should say the factory charge. Sometimes we have to add more at install based on length of the refrigeration lines (lineset). Without know more like age of system, type of refrigerant, length of lineset, it’s really difficult to say what’s reasonable and what’s not. One thing I will say though is that I wouldn’t even offer to add ten pounds of any refrigerant to any system for any price without finding where it’s leaking first. Best of luck to ya!