Like Ross said disk space is a lot more expensive than bandwidth.
Most of my servers are dual xeon's and are priced at around $225 or so each. They all come with two 120gb hard drives, and 2TB of bandwidth. One of those drives is taken and mounted as a backup drive for either daily or weekly backups. Then you have to factor in a loss of about 15gb or so on the primary drive to account for the space used up by the OS and various partitions. That leaves you with ~100GB of hard drive space usable on a server.
Now, remember that the server comes with 2,000GB of bandwidth. Divide 2,000gb and you'll get that each 1GB of disk space sold should come with about 20GB of bandwidth. Thats pretty much the standard proportion depending on the server you buy. Some come with 1TB of bandwidth instead of two, so figure somewhere between 10GB-20GB of bandwidth per month for every GB of disk space purchased.