I tried to compare common options quickly, taking into account replacement bulbs and the like, on a 200 gallon, and the Sky seems to be on "par" pardon the pun with T5's and Halides in terms of costs. The latter might be a bit cheaper over five years depending on how often you change the bulbs. If you have to buy a new Sky in five years that's where it may get significantly more expensive depending on if the T5 and halide ballasts and fixtures hold up or not. Skys also reduce or eliminate the need for chillers and the like though too... Lots of factors... electricity i am not sure is a savings really as to the light itself, maybe depending on the setup, say versus halides for example.After you take into account all the different factors, you will need to replace 1 watt of your current lighting with 1 watt of Sky lighting.
All and all, for how much coverage you get with a SKy, as an initial impression, i think Neptune has done a good job showing its a contender as to the lower to average cost factor for high end lights. There are definitely far more expensive light figures and LEDs out there than the Sky when it comes to how many you need to cover a larger tank .