Tight on Cash 600w vs 400w

There has recently been a large price increase in electricity gearing up for the world cup, its nearly Double the amount we are used to..

In our Currency it costs me 500 in electricity every month which is already quite allot.i am using a 400w which im sure uses at least half that amount.

This month i get a bill of 750...
Thats nearly a 30% increase..
The issue i am having at the moment is i am 4 weeks into flowering some awsome strains, and i want a good yield !!

firstly would there be much of a difference in yield with the 400w vs the 600w ?

also, how much would i be expecting to pay if the bill is at 300p/m for the 400w ? is there a big difference in the price?

i just need some advise on this before i go and buy a 600w light.. i want big yield, im just not sure if its going to be worth the extra money..

any help or advice would be great !

Ive heard you get something like one gram for every one watt your growing with. I could be wrong, thats what i heard.