RM1759 for 390x seems tempting...powercolor.

Anyone found any better deal?

Nah, the PSU only supply the amount of power needed by your computer hardware. The "850W" is the maximum rated power the PSU can supply, not the amount of power the PSU will supply all the time. Basically, for example if the components in your computer only need 400W, then the PSU will supply 400W.

Incorrect - the PSU will only supplies the required power.  In fact, lower power consumption often leads to better longevity due to less load on the PSU.

It's like this, if both have the same efficiency of 90% then:

850W PS ---> Use 250W ---> 250W divide by 0.9 = 278W from the wall plug @ 12.5 cents per hour (assuming 45 cents per kWh)
500W PS ---> Use 250W ---> 250W divide by 0.9 = 278W from the wall plug @ 12.5 cents per hour (assuming 45 cents per kWh)

It's the same 

The advantage of 850W PS is it can output more wattage than 500W unit.
How efficient is the power supply depends on how the power supply was built and what rating is has. 80+ Platinum, Gold etc and how it handles on varying temperature. You also have to take into account how efficient the power supply is when handling low power draws.
For example, The 80 Plus Gold standard requires 90% efficiency at 50% of rated load, and 87% efficiency at both 20% load and full rated load. So that means that if the 850W power supply is gold rated standard, it can deliver 87% efficiency from 170W onwards.

You need to know the amount of watts pulling from the wall. From the data above, I can conclude that the first bill higher because the amount of watts pull from the wall is higher than the second bill. During conversion (Power supplies convert one voltage to another), some energy will be wasted as heat. It's about efficiency. Efficiency describes just how much energy is lost as heat. Higher efficiency means less waste. The 850W PSU in the above example have pretty bad efficiency.

The 250W is the amount of watts output by the PSU. You forgot the amount of watts pulled from the wall to generate that 250W for the computer components. Power utility company bill you based on the amount of watts pulled from the wall.

For example, let say the 500W PSU have 80% efficiency rating & the 850W PSU have 60% efficiency rating. Which mean:-

850W PSU (watts pulling from the wall) = 250W / 60% = ~417W

500W PSU (watts pulling from the wall) = 250W / 80% = ~312.5W

Above example explained why the bill with 850W PSU is higher than the 500W PSU.