Mining consumes very less bandwidth. More importand is the latency
My Proxy is connected to ETC Mining Pool
www.ethteam.com.
And on this proxy is a 1x100MH/s windows miner running.
On the Screenshot you see 2 connection. 1 Connection is between proxy and miner. (local)
The second is the connection from proxy to stratum server. (internet)
My EtherProxy consumes 107 bytes/s internet.
=> Per Day: 8.8 mb
=> Per Month: 264.5 mb
Edit: Pool has high share difficulty (4G)