It has its own Wikipedia entry.

 Sinatra was not happy about the amount, knowing full well that even he was not being paid that much for the whole show. He accepted, however, that Presley's appearance would attract huge ratings for his show, something that his three previous specials had failed to do.

Presley's manager, Colonel Tom Parker, also made it very clear that this sum was for only two songs, approximately 6–8 minutes.Parker had hoped that showcasing Presley on Sinatra's show would re-introduce him to an older audience, an audience that would be less likely to forget him in favor of the next teen idol.

Quite the promotional effort - two-page spread by Hirschfield. You can see a larger version here.