It depends on how often the servers have people playing in them. Also, remember hardware requirements. It takes some hardware power to run multiple, populated, servers.
I have never had an active CS:S server, but I was running a 32 person DOD:S pub at one point. It was taking about 8 GB per day with the server full for about 4 hours per day.
Currently, I'm running a 16 person HL2-DM server and it's taking about the same.
Attached is my daily bandwidth usage report from my provider. Note that the "In" and "Out" values are reversed, since this is being measured from their switch. This is the bandwidth being used by a 16 person HL2-DM pub, which is full around 5 hours a day. I also attached a RRD graph I made myself.
gb.jpg (Size: 102.02 KB / Downloads: 36)
rrd.jpg (Size: 34.1 KB / Downloads: 19)