How much

  1. How much is it?
  2. How much does it cost?
    Do both questions mean the same?

Yes.

no

  1. How much is it?
  2. How much does it cost?
  3. What does it cost?
    Out of these three, which one is more colloquial?

None of them are more colloquial. They’re all standard English.

How can we express this idea colloquially?

What’s it cost?

“How much will this set me back?”
“What’s the damage?” (This is used after a service has been performed.)
“How much do you want for it?”

Beesneees,

  1. What’s it cost?
  2. What’s its cost?
    Which one is correct?

The first one is correct.

I mean :

  1. What’s it cost? = What is it cost?
  2. What’s its cost? = What is its cost?
    If #1 is correct, explain to me its grammar.
    What is ‘cost’ here. I believe it is a noun.
    Thanks.

It’s an abbreviated – and therefore more colloquial, conversational – form of ‘What does it cost?’

That particular -'s abbreviation is only used in informal speech.