It depends on how much slot each server is. If all servers are around 40 slots and are popular, he will run out of BW and users will see lag.
My Euro COD4 server in itself pushes around 4-8Mbp/s. If all his servers are full, players will see lag since he will be pushing his BW limits. ET is uses almost 2x BW then COD4.
Although each data centers tells 100Mbps/port, you never get that much in real time. If you get 80Mbps its good enough.
I have seen GSP's providing 64 slot in 15$. They also say they have resourced but then there resources come from 'swap' instead of 'RAM' and they also 'auto kill' any server which uses more then 30-40% CPU. I have seen many host don't even provide 128 hunkmegs. I have seen GSP's hosting 300-500 slots one 100MBps port Now they do make profit because there service is cheap and every time they get newbie admins to host with them because they 'cheap'.
It all depends how GSP's run there hosting. More or less you also need to consider, clients don't stay forever. There clan dies and they move on or sometimes there is big network failure or HD failure and they move on because they don't like few hours downtime or so once in a year. At that time you will end up paying more to keep your machine and then additional charges to fix things.
There overall margin is very less unless you start stacking servers on one machine.