Sounds like it was/is a well cared for, one-owner 04 snake. Coupe or 'vert btw?
Make sure it's the real deal of course (verify VIN through SVT, supporting paperwork, hopefully no prior wrecks, damage, clean title, yadda yadda; in other words ALL of the intangibles better check out for that asking price IMHO).
Is it worth $24k?
Dunno, but some will say it's priced high just because this is a mustang enthusiast message board (LOL).
You know how that goes...
However, if you've been patient, done your homework, and searched high and low for a pristine 03/04 Cobra for several weeks/months and none have come close to this example you are considering, then give it strong consideration.
Be sure to look through the classified sections on this message board as well as several other mustang message boards to compare what other 03/04 Cobras are selling for so you'll have a better feel for the Terminator market. These cars do hold their value well, but higher mileage ones can be purchased in the mid-teen ($14k - $17K) range IIRC...could save a few grand by buying one with a little more on the odometer.
Personally, I wouldn't purchase that '04 for $24k because like you said, you are into new 5.0 mustang territory pricing (and what an engine those cars have, oh my!).
G'luck !