About how much extra each month is it to send a lot of e-mails and IMs when on a Cingular contract?

About how much extra per month? My job requires me to send a lot of e-mails and IMs, and I'm trying to decide which company to go with. So far I seem to like Cingular, but no one has been able to tell me about how much extra per month I should be expecting to pay.