It greatly depends on the machine and usage. They more you use it, the higher costs per year but it gets cheaper and cheaper per CF pumped.What are the annual cost of owning your compressor, for parts and maintenance?
Some machines are cheap to buy and expensive to maintain
Some are expensive to buy and also expensive to maintain
Unless you find something like an old k14-85, you won't likely find cheap and cheap together.
I only reference that one machine because they were ridiculously overbuilt for their low speed and low outlet pressure. It is nothing to get thousands and thousands of hours out of final stage valves. I have one of them with 7000+ hours on it and it has never been opened outside of final valves. It has been on rental in a local dive shop for the last 18 months just humming along every day.