Yup - you are 100% correct. Power costs about NZD$0.22 per kWh when weight averaging the day and night rates. This means I use:
18KW * 24h/day * NZD$0.22 = NZD$95/day
Meanwhile, I could sell the Gridcoin for about NZD$0.04 per coin, so I earn about:
800 GRC/day * NZD$0.04/GRC = NZD$32/day
That being said, recently GRC traded for almost NZD$0.15 at one point, which would have yielded NZD$120/day.
In reality, there are still several reason I continue doing this:
I need the hardware for my research anyway, and this is running on idle cycles. The power bill of using those cycles is only a fraction of the total compute powerbill.
The price of GRC will go up, so I will see a good return in future if I were to sell
I have plans to use the GRC I mine on idle cycles to commission compute in the future through the BOINC and GRC network, if I can overcome the technical hurdles of the models I work with.
There's always a blessing for selfless services but the selfish can never see or understand it.
Good luck in your endeavors.