If someone escapes scot free, they avoid payment or punishment. 'Scot' is an old word for a tax, so it originally referred to avoiding taxes, though now has a wider sense of not being punished for someone that you have done.
When people say this, they either want to know how much they will be paid for something or want to see evidence that something is valuable or worth paying for.
(USA) This is used to describe something that is deemed worthless. "He's got a Ph.D. in Philosophy." "So? That and 50 cents will buy you a cup of coffee."