I've heard before its about a meg a minute in terms of data, obviously how much that costs depends on your netowork, data tariff etc. Just add the per minute Skypeout charge to that for who you want to phone and you'll have your answer
My name is Roy Timor-Rousso and I am on the fring team
when doing fring to fring call or fring to Skype, MSN, Gtalk etc over 3G / GPRS/ EDGE than fring consume about 5-8 MB per full hour talk time (or ~80K-130KB per min) and about 10KB per hour is idle mode (we keep the presence information on your buddies list and your statuses to other accurate)
(for fring to PSTN/GSM via SkypeOut or SIP, you need to add the cost of SIP or SkypeOut per min)
I don't have the number for VodaCom and MTN for data packages; yet taking some examples from UK and IT you get an idea about actual cost
(see example in our site at http://www.fring.com/about/cost_saving/ )
BTW: we see more flat rate plans like 5 UK pounds from 3 in the UK