139% per annum, made a mistake most likely in the currency conversions.

21450 ZAR for mining machine. With difficulty and price relatively stabalized the machine is making about 82 ZAR per day. That's 29930 ZAR per year or 139% per year. That is assuming price does not rise further, then the scenario is even better since there is a lag time to get new mining equipment online.

(Not included here is the devaluation of the machine over time or the larger amount of BTC it generated early in its live)