In Philadelphia to kick off his $10-million "Glass Spider" tour, David Bowie had reporters grousing Monday when he served burgers before the press conference began. Many written accounts of the gathering pointed up the irony of the tour's expense and the homey fare, but Bowie himself was undeterred. "It's the most extraordinarily expensive tour that I've ever done," the chameleonic rocker said. "It has elements of mixed media . . . film footage, choreography, minimalism, symbolism . . . oh, and there's some rock 'n' roll." By the time the tour ends this fall, Bowie will have performed more than 100 shows in 15 countries.