How much is the hat?
- It is 10 dollars.
How much does the hat cost?
- It costs 10 dollars.
What is the price of the hat?
- It is 10 dollars.
How much is the price of the hat?
- It is 10 dollars.
How much do I owe you about the hat?
- It is 10 dollars.
- You owe me 10 dollars.
(Are the questions and answers correct?)
The only thing that is wrong is the last question. It should read "How much do I owe your for the hat?" , not ABOUT the hat.
The question before that is rather awkward, and would mark you as a non-native speaker. It be better to say simply "How much is the hat?" When you ask "How much.." it is understood that you are talking about price, so you don't have to say "the price of".
Please note: DrWLS meant to type "How much do I owe you for the hat?"
(Reversing "you" and "your" is a very common typo that many of us make. I have to watch myself, too!)