Thanks to the nice people here on Geekzone, I found out that Vodafone were ending their prepay users transferring to the Motormouth plan on prepay.  Hmm, I thought, something up here, so headed to DSE to buy a connection pack and changed it over to Motormouth (text the word Motormouth to 756 after having set up your account pin).

However, smelling at rat, I decided to do a little maths.  There are a few 'supposes' here, everybody has a different call distribution but I decided to just choose one that seemed ok. 

Suppose a person calls half land-line and half cellphones.  For the calling cellphones let's suppose it's equally half Telecom and half Vodafone.

Calculating:

Motormouth:
land-line 0.5 * 49c/min = 0.245

Cellphone VF 0.25 * 49  = 0.245

Cellphone TC 0.25 * 139 = 0.3475

Total averages 71.5c/min

Superprepay

all calls cost 89c/min.

But wait, there's more.  Super Prepay rounds calls up to the nearest minute, but the old 'classic plans' round to the nearest second.  So, let's suppose the average call is 5 minute (I'm guessing here, my calls are always shorter but I wanted to be charitable) so you are looking at a rounding up of an average of 0.5 minute which is 10%.  Shorter calls have a higher rounding percentage.

So 10% rounding up of 89c/min = 0.979, which is 37% higher than Motormouth.