how to determine someone's copywriting skill? | warrior ... - how do i determine the ph of skin care products

Hi guys, I have a few questions about the sales page.
How much do people expect to pay to write a high-quality sales page?
How open is copywriting to sensitive areas of customers (like not needing to look like spam )?
How do you determine if a writer can post as they say?
Also, do you have any arrangements to ask for performance pay?
In other words, if the sales page author claims that they will get the conversion (assuming the product is of quality), let them do some sort of transaction, charge a small fee in advance, and then get a bigger bonus when the page is sold, okay?
I have never worked with another copywriter for my product before and I need some help and advice. Thanks.