I have instant gas too. I love it coz it only heats AS you use it. No heating at a certain time to only be half used.
I pay $140 for a bottle (tall skinny one, I think 90kg, but not 100%) about every 8 weeks. 4 kids & me, 2 little ones can have 2 showers a day for 20 minutes - half hour each My stove/oven is also on gas.

Ok, my last power bill over East, with hot water on off peak 2 was I think about $700 - 800.
My last power bill here was $186. Add to that the 1.5 gas bottles for the quater & thats just under $400. Not being careful, cooking for work (so gas stove running a few hours a day). & with my R/C heater going overnight for about half the last quater.

I think that works out pretty ok