How is it rude to ask someone how much something cost?

It depends?, Are you really interested in purchasing the item, Even if not its Ok, But don't walk all around asking about everything with no intention to buy the stuff. and don't ask about lowering the price without the cash in hand. DON'T be afraid to ask for the price to be lowered if you feel it is too high, Even in stores.

However, it is not considered polite to ask someone how much they paid for an item they are wearing or have in their possession.