Because the price that you found for the Powell headjoint is the price for the Boston, Philharmonic or the newer one which is called a Soloist.
The Signature sytle of headjoint is the one on the Sonare bodies which new is almost half the cost of the 3 mentioned above.

I really don`t know why the Signature one is so much less in cost. It is silver like the rest of them. Mabye the company kept the price down on it because it is being offered on more the intermediate flutes and not their hand made flutes?

The Signature headjoint is also hand cut.

Yes they do. I talked to Sara Kelly that works for the Powell Company who told me this.

It is on a Sonare body, but is a Powell headjoint, not a Sonare headjoint. It is called a Signature style Powell headjoint.