Like more than you can imagine.
A Metlij just at cruise power, not even max, will drink 60-70 gallons per hour.
Avgas is $7/gal, and also not even high enough octane for a Merlin.
You’d be at $500/hr just in gas.
Undoubtedly in the many millions of dollars per unit purchase price category, so a whole other thing than something that's cheaper than a Cirrus SR22.
He built "The fastest single engine turbine plane in the world"
https://www.youtube.com/watch?v=5pUpJz6QbBk
It was "only" just over 1000hp.
It didn't end well (but at least nobody died):
https://www.youtube.com/watch?v=swo_FHGkfCM
I'm 100% certain "full military power" aircraft are well outside the set of compromises general aviation pilots are prepared to accept. (And probably outside what the FAA would allow you to build and fly?) I wonder what the service 8intervals were on a 1490hp Mustang? (Did they even bother with "service intervals" on planes that probably didn't come home often enough to make it to the 2nd service?)
"For some airplanes, such as the P-51D, use of WEP [war emergency power] required that the engine be inspected for damage before returning to the air.[4] 5 hours' total use of WEP on the P-51D required a complete tear-down inspection of the engine.[4]"
Considerably more than a mere 1500hp, though, by at least some accounts. https://en.m.wikipedia.org/wiki/War_emergency_power
No idea why that old story came to mind just now. He wasn't hurt, if anyone's wondering. Of course, it was a pretty shallow ditch.
Still quite an achievement, but would be nice to see a more authentic engine option.