Best way to ask the user how much something cost, approximately?

1. How do you ask how much it costs?
2. How do you ask people how much they are willing to pay?

How do you ask how much it costs?

"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).

How do you ask people how much they are willing to pay?

Willingness-to-pay: Open-ended

This one is as straightforward as it sounds. After presenting your product/service concept, ask respondents how much they'd be willing to pay for the concept, and leave it open-ended so they can type in whatever answer they want.