How much would it cost to take a 5-pound Maltese on an airplane?

Please tell me how much it'll cost to bring a 5-pound, friendly Maltese onto an airplane going to Orlando, FL from Las Vegas, NV, using United Airlines on July 10th, 2011. (It's a one way trip because then we're going to Chicago and then Traverse City, MI.) But, anyway, she's very sweet, and shouldn't be much of a problem at all. I have it all planned out (her walks, etc). (We are going to buy an airplane crate soon).