I had a routine AC system check. the system was cooling just fine. The reps were here 15 minutes total. They said they had to put in 4 lbs of recycled r-22 refrigerant. they charged me $125 per pound or $500. How do I know it need that much refrigerant? It was cooling fine before they came.
You could refuse to pay the bill until they provide you with documentation of the pressure levels that prompted the addition of refrigerant and the amount of refrigerant required. They should have recorded this on the service sheet. Ask for a copy of their service sheet. Also, justification of their price of R-22 and why it is $125 per pound.
I believe HVAC contractors are required by the EPA to meticulously document all exchanges and uses of old refrigerants like R-22.
They should have discussed all of this with you BEFORE they did any repairs or additions of refrigerant. Especially since the cost was going to be so high.
R-22 is being phased out, and manufacturers are only allowed to produce limited quantities. As the quantity produced goes down, the price goes up. $125 per pound is not unreasonable. It's a bit on the high end of pricing, but not excessively so.
That being said... Your A/C is a closed system. Meaning that under normal operation, no refrigerant should be lost. If your system has lost 4 lbs of refrigerant, then there must be a leak in the system. If the HVAC company did not repair the leak, they'll be back soon to sell you some more refrigerant.
As others have pointed out, they should be keeping accurate records regarding the usage of R-22. So they should be able to provide you with proper documentation showing how much refrigerant they used and why.
Without knowing the make and model of the equipment, the gauge pressures before and after the system was charged, the ambient outdoor temperature, etc. It's impossible to say if the system needed refrigerant, or if they actually added any. I'd contact somebody higher up at the company to discuss what happened. If that does not yield satisfactory results, contact a lawyer and/or the BBB.