I've been using gas heater for 8 months now,.. €1.20 - €1.50 a day is what I spend on gas,.. even before Xmas with the big rush of work the bottle lasted just under 3 weeks.
It won't save you money,.. but it might help you work faster and be able to do 1 or 2 extra jobs at the end of the day, increasing your profit that way.
It depends on your round really,.. there are big advantages on irregular work and first cleans, but on fortnightly work you'll not see a huge difference in speed of cleaning.