They bought it for $3.5B, but the annual revenue of SA Power Networks is around $600M. I don't know the profit margin, but they won't have to wait for 200 years to get their investment back.
This just seems like a really bad deal for whoever buys it.
Regarding your 10% assumption: In June 2014, the Adelaide Advertiser reported that SA Power Networks made after-tax profits of $420 a year from each customer compared with $92 for another Li Ka-shing-owned utility in Britain. I don't believe the average customer paid $4,200 for energy in 2014, so their profit margin must be much higher than 10%.