Would you buy a new AC unit or pay $500 annually?

[Allen Wu]

Hi All - I have a rental property closed back in 2015. The AC unit was installed in the late 1980s. The Unit now requires annual maintenance (Freon refill and tune up). The cost has been going up everything year. Recent cost to do this every year is $500.

Given the age of my AC unit, would you keep paying $500 every year (or more) to maintain it or would you just buy a brand new AC unit that will cost ($8k)? 
Appreciate your insights. 
Note: it’s a condo. Central air unit.