To add to what's been said, and give a real world example of a highish miles 3.4 996, I've owned mine for a few month now. I went into ownership with eyes open having had one ages ago.
I bought a 100k miler with an oil change for every year and a very fresh interior but not much else spent on it. I got it cheap, knowing it would need TLC..
To date I've put a low temp thermo , discs pads and hand brake shoes, full service including tip oil and filter change drive belt and coil packs replaced, a header tank and radiator, ballast resistors, various brake lines, battery, spare key, number plate plinth, oil pressure sender, bumper mesh, third brake lights (full units)
I've prob spent a third of the purchase price on the above (including labour, it would have been a lot cheaper if I'd the skills to get oily) and I know the are various suspension parts and other bits yet to do.
To add balance, a pal bought a well sorted 3.6, similar mileage for a good few grand more, but has had to do absolutely nothing else to his one yet. My totals are now approaching his purchase price at this stage.
With the mileage we're prob both looking at an engine rebuild in the next few years depending on the roll of the dice , but then when did rebuild become a dirty word? I've had a number of engine rebuilds on other 'reliable" marques, it was just looked on as part and parcel of the joys of ownership...
Also worth mentioning that even with the bits yet to be done on mine, and a possible big engine bill, it's still way way cheaper than any other air cooled 911 out there, and a whole lot cheaper than any properly sorted post 996 911.