I can't imagine a royal doing anything for money, even 10 Million; there would have to be some personal reason for him to take the offer. Maybe spiting the entire royal family over some disagreement, or wanting to have the most famous dick in the world, or something similarly ill-conceived. I could imagine someone 18 years old doing something like that, but not so much someone 28 years old like he was at the time.
Whatever the reason, let's say he hits his head and loses his ability to tell a good idea from a bad one, the Royal Household would probably catch wind of it and have the whole thing shut down. The tabloids would have a field day, but as long as the tape isn't made (or at the very least, isn't distributed) the story would die down in time. If a video is distributed, then Harry would be ostracized by the rest of the family and skewered by the tabloids for the next decade, a bit like what happened with Sarah, Duchess of York when she had a compromising photograph of her published in 1992.