150-500 is too much but you are paying for the knowledge and special tools.
1st of all, newer cars use A134a as refrigerant.
2nd, there could be a number of reasons that the AC is not working. lack of refrigerant could be one but unlikely for 3-year old car.
I'll check the following before paying someone to do it:
is the compressor engage when u turn the ac on?
is there enough pressure in the system if it is on?
If you don't know how to check these, maybe you should pay someone to do it.
Good luck
