I was just doing the calculations the other day on prepaid vs straight up contracts and it was very interesting.
For a straight up 100, you get 100 each for minutes, sms and data bundle: for prepaid you just need to add the R49 all-in-one voucher on top of your R100 airtime voucher and get your 100 sms's, data bundle and also get 100 Cell C to Cell C minutes. So, for a R149 prepaid airtime you get the same as a straight up contract plus the bonus of the Cell C to Cell C minutes.
For R289 (R89 all-in-one voucher included) prepaid airtime, you get 200 of all the straight up 200 contract and, again, an extra 200 Cell C to Cell C minutes.
If you already have a phone or intend to buy it outright you have a great deal either way.
I am still contemplating whether to go contract or not after looking into these prepaid costs.