"How much does this cost?" "How much is this?" "What does this cost?" Replace 'this' with 'it' if you're already talking about the item. You could throw in 'exactly' if you wanted ex. "How much does this cost exactly?" or "What exactly does this cost?" ('exactly' can go in more than one place). It's not impolite to ask directly. I wouldn't say anything longer than "Could you (please) tell me how much this costs?".

If you were buying something at a store, say something like: Could you please tell me how much this costs? If you were asking a friend or stranger about something they own, say something like this: That's a very nice ______, I wouldn't mind having one. How much did you pay for it?

If your friend already bought the item, you would ask: How much did it cost? Do you mind telling me how much did it cost? Here is an indirect way to ask... "Wow! So nice... It must have been expensive! " Most people would answer: "No, it was only $xx" (only if it was not expensive.) But if it was expensive, your friend may not feel like saying the price unless prompted again: You could indirectly ask... "I hope it wasn't too expensive...?" (ask it like a question :) Just some ideas :)

One slight correction: "...how much it costs?"