I must say, you've been lucky. All of our estimates towards the end were ludicrous. The last month was twice the amount we were billed for during the previous month. That's what got us to change.
I never really expected to save using a prepaid meter. It simply allows more immediate knowledge of how much is being used. In Cape Town, at least if you get from the city, the units cost the same.
I paid R 600 for an electricity voucher today of which only R 469.83 was used for actual electricity (434.2 kWh). The rest is VAT and a service charge.
I use about 20 kWh of electricity per day. I am one person living alone.
AFAIK, Prepaid and postpaid cost the same, price differences are based on usage patterns...
If you obey all the rules, you miss all the fun.