Can anyone explain to me why this statement is incorrect: "I wonder how much would it be ..."

How much would it be? is a question. (1) would only make sense if punctuated to represent speech, where the speaker muses "I wonder..." and then asks the question.

"I wonder... How much would it be...?

I’d use ‘cost’ instead of ‘be’ with ‘how much’ generally.

Otherwise both would be correct but favoured differently depending on context and stress.

For 1. You wouldn’t necessarily be expecting an answer (at least an exact one) It’s more a speculative question that would include what you are talking about in the second half. ‘It’ undefined until the rest of the question. How much does it [cost] (to buy a Mars bar)? - 70p I wonder, how much would it [cost] to (buy a Ferrari)?‘ - too much

For 2 the topic is in play. The it is defined and the sentence concludes naturally straight away. ‘We should (activity)’ ‘I wonder how much it would [cost].’

Wondering about cost apropos of nothing feels strange so 2 feels more comfortable as you are referring back to the thing being discussed. 1 is still fine though.

I think for us, non-native speakers, it is best to realize that you are actually inserting a question into a statement. You would not say:

I wonder where does he live.

but

I wonder where he lives.

This seems to be the same, therefore you need to change the word order:

How much would it be?

becomes

I wonder how much it would be

The first example is perfectly normal if you are in a jewelry shop and ask the proprietor, who had just told you the cost of a bracelet:

I wonder how much would it be if this clasp were replaced with one like that one over there.