Want proof that Madonna's chief appeal is now to the middle-aged? How about the $300 ticket price for some seats at her Re-Invention Tour? Apparently, the 45-year-old singer has counted on a lot of folks who have a lot of disposable income to attend her concerts. And she was right to do so, Billboard reports. The 55-date tour, which launched last week in Los Angeles, is nearly sold out, and Billboard estimates it's on track to gross $120 million worldwide, already making it the front-runner for the most lucrative tour of 2004. That's a figure that would far outstrip the earnings of Madonna's similarly popular Drowned World Tour of 2001, which grossed $75 million from 47 sold-out shows. Some 920,000 ticketbuyers are expected to pay to see a spectacle that includes (in addition to Madonna and her dancers): acrobats; skateboarders; three video screens; and four set changes mounted on a turntable stage that whirls around at 15 miles per hour. ''This is a show, not a concert,'' tour production manager Chris Lamb tells Billboard. ''This is more theater than rock 'n' roll.'' Because mere rock & roll wouldn't command as high a ticket price.