kane

Tours are put on by promoters. A promoter works with the artist and the artist's management to determine what size venue they can fill and how much they can charge. The promoters either then pay the act a percentage of the ticket sales or a flat fee per show. Album sales really has nothing to do with it. If there is a demand for the band to play live they can put together a big tour. A couple of good examples would be the band Wilco and the band Phish (before they broke up) these bands sell an decent amount of records, but they get very little airplay and no video play and have little promotion other than word of mouth and album reviews in magazines. When their new albums come out they will debut high, but sink fast because they have a rabid fan base that goes out and buys it immediately or they download it. But when the bands tour they sell out places that normally are reserved for much higher selling acts. The reality is a larger percentage of their fans want to pay to see them live than the normal act. On the other end of the spectrum are people like Kelly Clarkson. She had several huge hit songs and a record that went platinum multiple times and she had to cancel her first tour and fully revamp it. They thought she could play 3,000 - 5,000 seat places in smaller cities and 10,000+ seat places in larger, but nobody bought tickets and she ended up playing 800-1000 seat places.

So really album sales has nothing to do with it. If a band sells 500,000 albums, but the album is downloaded 5 million times, they are probably going to do very very well touring. If the same band just sold the 500K but didn't have any downloads they will most likely be playing much smaller venues.