How much would an item cost that turns you into a half-dragon?

[Elephant]

How about a Ring of Shapechange?

Command word, 5 times per day, lasts nearly 3 hours per activation. 9th level spell, 17th caster level, cost: 276,900 gp.

It drops to 56,580 gp if you make it usable only once per day.

 

[Creamsteak]

(I was wrong, you do indeed get the breath weapon. I'm just blind.)

An advantage the Ring of the White Wyrm has over a ring of shapechange is (I believe, could be wrong here), Shapechange causes items to morph into your body. The Ring of the White Wyrm allows you to keep all items functioning normally.