Is $4,320 to replace AC unit a fair price?

I bought my home a little over a year ago. My AC unit worked perfectly great all last summer, except for one time a line temporarily froze. Up until I shut it down for the season, I had no issues. When I started the unit this season, it would not blow cold air. I did troubleshooting but couldn't get it to work, so I called a technician. They informed me the unit was fully out of refrigerant and is 29 years old, using R-22. I was curious how it lost all refrigerant over winter if it was working well. I was concerned about being upselled but believe the age and refrigerant type justify replacement. I have approved the replacement and they will come next week. I received a $380 discount for paying cash or cheque. My house is small, under 700 sq ft.