How much does a big diamond cost?

Big is relative.

As an example, Elizabeth Taylor wears the Taylor-Burton diamond of roughly 69 carats in the photo, below. For most women, this stone is about as big as one might want to wear, although Elizabeth Taylor was only seen wearing it in public once. She feared is loss or theft.

After their divorce, she auctioned the stone and it fetched US$5,000,000 in 1978. A current value would be higher.