Let's cover a couple of fine points about refrigerant gasses before you spend your money.
1. Refrigerant gasses are used in a 'closed loop' condition. It doesn't wear out, it can only leak out.
2. In some cases, if you look at the gas pressure on the low-side only, a dirty condenser coil will indicate a need for more gas; but this is a false reading.
3. Continuing to add gas to overcome the dirty condenser coil can eventually biring the systeme to an explosive conditon, with the hot gasses running at above 400 psig.
4. There is a wise saying in air conditioning work; "Clean first, gas last". Keeping the condenser coil (the outside unit) and the evaporator coil (the inside unit) clean will keep the unit working at peak efficiency without the constant addition of refrigerant gasses.
5. Remember, it doesn't wear out, it leaks out. If indeed your refrigerant is low you need much more than a recharge; the leak needs to be found and repaired.
I hope this can have some effect on hiring novice air conditioning service men who make tons of money on the statement, "It just needed a little gas". The vast majority of air conditioning repairs I have made are (1) cleaning and (2) reconnecting a burned off wire.