Comparisons with the word "twice" — i.e. twice as expensive vs. twice more expensive

Question

Why is it that it's okay to say "This dress is twice as expensive as this jacket" but not okay to say "This dress is twice more expensive than this jacket" ?

Furthermore, it seems okay to me so say, for example, "This book is two times more expensive than that dictionary" though "This book is twice more expensive than this dictionary" seems wrong.

Any ideas why?

Answer 1

"This dress is twice more expensive than this jacket" would mean that the dress was more expensive by an unknown amount twice, which would be the same as simply saying that the dress was more expensive than the jacket.

For similar reasons "This book is two times more expensive than that dictionary" is not good.