I'm in the UK so my experience will be a little different.
I use a 14kg bottle.
I got 6 brews out of the last one and it's not dead.
I have one in my patio heater so can swap over if I need and I also have a 5kg bottle for camping which can always help me finish a boil.
My burner is a lot smaller in BTU than most. 27000btu but it does the job and boils up in under 30 minutes. I don't think I could scorch my wort if I wanted.
Refills cost me £14 (I think)