Hmmm
As far as chickens, I think the most expensive we've had so far was probably one of the birds we got at the Ohio National. We got a blue orp hen and a blue ameraucana cockerel (I want to say we got a black ameraucana pullet too, but I don't remember). I don't remember how much we paid, but probably around $20.
My "rarest" is probably my cookies and cream pullet from Meyer
The cookies and cream are a hatchery exclusive hybrid, so you can only get them directly from Meyer. Not super rare, but I can get any of the other "breeds" I have from other sources.
The rarest and most expensive bird I've ever owned is my orange winged Amazon parrot. We paid around $600 for her 25 years ago. She currently lives with my MIL.

My "rarest" is probably my cookies and cream pullet from Meyer
The rarest and most expensive bird I've ever owned is my orange winged Amazon parrot. We paid around $600 for her 25 years ago. She currently lives with my MIL.