I'm thinking about offering my best friend money for a sexual favor.
Why do you want to offer your best friend money for sexual favor
I'm feeling bored and I want to see how they react.
You are treating your best friend like a whore. If you want to take the relationship to the next level, then have an honest conversation with your friend or your friend will be highly offended by your proposal.
I'm pretty sure my friend will be into it though.
Regardless of whether your friend is interested, offering someone money for sex is illegal in most places. And if your friend isn't interested in it, you're likely to upset them and they may end the friendship with you. Find a more constructive way to deal with your boredom.