Er, obviously buying cash is going to work out cheaper over-all than buying over 24 months. The question is, is it cheaper enough to make it worthwhile shelling out the cash upfront?
Personally, I think buying cash is always better, however, the cellular companies worldwide have proven that most people disagree. (Hence buying R 10 000+ cell phones on contract, etc.)
A good comparison would be - how does FNB's contract prices compare to Vodacom et al? Especially considering you don't get data etc. bundled in. Although there are usually extra discounts with FNB which would apply to many buyers as well.
There are of course exceptions. Currently paying a handset fee on my Cell C contract for my S4 of R299pm - over 24 months that would've cost me R7176. I got one of the first S4's after launch and cash price then was over R8k - so the Cell C deals are often bargains!