At some point, you'll be so big that most large ISPs will want you to install cache nodes inside their network. You - as them - will both save cash because you

At some point, you'll be so big that most large ISPs will want you to install cache nodes inside their network. You - as them - will both save cash because you can relie less and expensive transit traffic and the ISPs customers are more happy with the service. So - all in all, the ISP usually ends up with most of the bill, but that's okey for them, as their users want to access this content anyway. This is not some scheme to "get rich fast", they have just figured out what all other CDNs do, and found a way to game the system to become more relevant quickly.