I guess the right sentence would be " The dress is very expensive".

句子本身没有语法错误，如果你是想表达，这个衣服比另外一件贵的话。但习惯上使用more， 就是“The dress is more expensive than……”

I agree with Carlos. If you want some more examples (a few more descriptive) you could use: The dress is extremely expensive. The dress is really expensive. The dress is insanely expensive. The dress is crazy expensive. The dress is way expensive. The dress is super expensive. The dress is exceptionally expensive.

The dress is very expensive or The dress is so expensive

Correct while you're comparing the price of the dress to that of another subject.