Special mechanics, special pilots/training, and most expensive - special fuel delivered by aerial tankers in faraway places. If you wanted to track the SR-71 you might follow the KC-135 tanker carrying its special fuel and wait for the blackbird to show up. Knowing this, I suspect the AF would have a whole fleet of KC-135's full of the special fuel in the air all over the world, with only one or two actually meeting up with an SR-71. Expensive enough yet?
Also, I believe improvements in sensing and anti-aircraft missile tech made it increasingly dangerous to fly over hostile territory. Maybe there's still a need for recon, but is a manned aircraft still needed for this job?
OTOH I have to agree that decommissioning the SR-71 was a mistake. Beautiful aircraft, unique design, still the fastest thing in the air. It should have been kept flying, but for pure research into the science of speed. If they quit trying to keep it so secret, a lot of the expense would go away.
We will have to agree to disagree: At the time the SR-71 was decommissioned there was nothing that even got close to shooting it down. To compare the cost of operating a totally successful aircraft to developing a new one isn’t even close... as far as the special fuel...why wouldn’t the same qualities be needed for the newer aircraft if it is going to operate in the same environment? In my opinion the SR-71 was decommissioned for political reasons. I have nothing to offer as proof other than the liberal policy of F&*king up any military program that is successful!!!