If someone offered to pay me to DM, I'd have to set up sound effects, ambience lighting, props, character illos, and a soundtrack. I do too much off the cuff in my own games, and if I were getting paid I'd feel compelled to do more.
Of course, the reason I run my own game is because I love telling stories in my world, and I like seeing how the players direct the stories. If someone wanted me to run a pre-made adventure in a setting I had no great interest in, I . . . I'd probably do it for $20 an hour.
And for a small fee, you can buy extra magic items or HP for your character. *grin*
Where's Bugaboo when ya need 'im?