It not the material. The Ti shell itself isn’t going to cost more than a PF Ti shell.
It’s threading that shell that takes time and money. When T47 came out taps for threading were very expensive. Even now, left and right Park Tool taps are $700.
Titanium is very hard so they may even need higher quality taps than Park sells. And those taps are also going to wear out tapping titanium over and over so there is a replacement schedule.
On top of that you have the labor.