The purchase price is the beginning of the conversation, not the end.
The number nobody quotes
When people talk about air-cooled 911s, they talk about purchase prices. What a clean SC costs, what a 993 costs, what the market is doing. They rarely talk about the running cost, which is the number that actually determines whether you can live with the car. An air-cooled 911 is not an expensive car to run by exotic standards, but it is not free, and going in with a realistic annual budget is the difference between joy and resentment.
The annual service
An air-cooled flat-six needs regular service whether you drive it much or not. A basic annual service, oil and filter, inspection, the small adjustments these cars need, runs a few hundred to around a thousand dollars at a specialist depending on what it includes. A more comprehensive service with a valve adjustment, which these engines need periodically, runs higher, often one to two thousand. Budget on the order of fifteen hundred dollars a year for routine maintenance on a car that is fundamentally healthy, more in the years when a larger service comes due.
The things that are not routine
Then there are the items that are not annual but are inevitable. A clutch eventually. A top-end engine freshening at some point in a long ownership. Suspension bushings, brake refreshes, the fuel and cooling system attention that an old car needs. None of these come every year, but over a decade of ownership they add up, and a sensible owner sets aside money against them rather than being surprised. A reserve of a few thousand dollars a year, accumulated, covers the larger jobs when they arrive.
Insurance and storage
Collector insurance on an air-cooled 911, on an agreed-value basis with limited mileage, is relatively affordable, often in the range of several hundred to over a thousand dollars a year depending on the value of the car and how much you drive it. It is far cheaper than insuring a modern performance car as a daily driver, which is one of the quiet advantages of collector ownership.
Storage is the variable that depends entirely on your situation. If you have a garage, it costs nothing. If you need climate-controlled storage, it runs anywhere from one to several hundred dollars a month, which over a year becomes a meaningful line item. An air-cooled car kept outside or in a damp space deteriorates, so storage is not optional, it is just a question of whether you already have it.
The honest annual figure
Put it together and a realistic all-in running budget for an air-cooled 911, assuming you have your own garage and a healthy car, lands somewhere around three to five thousand dollars a year, covering service, insurance, a reserve for larger jobs, and the consumables of actually using the car. Less if the car is healthy and you drive little. More in the years a big service or a repair comes due. If you need paid storage, add that on top.
That number is the one to know before you buy. The car that costs fifty thousand to purchase costs a few thousand a year to keep, and a buyer who budgets for the second number as well as the first is a buyer who will still be enjoying the car in ten years rather than selling it in frustration in two.