That electric price is really low so you can mine on 290/390 hardware and it won't make much of difference in your power fees.
The 290/390 cards are a beast when it comes to hashing. On my 290s they are refurbished models and the core clock is too slow to take full advantage of the vram and utilize about 80% of it, over clocking I can get 29mh easy and 90% vram utilization.
So no you are not missing anything, people have moved to these cards because of power consumption and high rates. You can get similar hash with modded bios and lower watts.
The 290/390 cards are a beast when it comes to hashing. On my 290s they are refurbished models and the core clock is too slow to take full advantage of the vram and utilize about 80% of it, over clocking I can get 29mh easy and 90% vram utilization.
So no you are not missing anything, people have moved to these cards because of power consumption and high rates. You can get similar hash with modded bios and lower watts.
the only limit of using the 390 is that if you have the total limit of the power consumption, you cannot get high hash rate.
I've $0.15/kWh so that's why I use RX 480. 
