its not worth it for two reasons:
- server hardware costs are such a minor part of a hosting package it hardly makes sense to upfront that cost unless you are talking a massive server farm
- more importantly, if you buy your own servers and co-locate, YOU have to support the server which means given a disk failure, getting your server back online could take days or weeks. Even if you have software issues, co-location does not include someone fixing your buggered linux or security installs