Just made a 5gallon extract batch today (started at 6.5gallons), from beginning to end (60min boil) used 2lbs 7oz of propane, one gallon weighs 4.2lbs, so I average about 8 beers per 5 gallon tank.
I use a SQ14 bayou classic burner. Not the fastest, but doesnt burn wort and seems economical...