I just purchased a Blichmann floor burner, along with a natural gas conversion kit. I have a gas line outside where our patio will be (coming in spring 2013!), and figured that would be a convenient location for brewing outside in the summer. Out of curiosity, I looked into the cost of propane versus natural gas. Here's what I came up with:
20lbs of propane / 4.11lbs/gal of propane = 4.87 gallons per tank
4.87 gal x 91,690BTU/gal = 446,180 BTUs per tank of LP
Cost to refill a tank..? $20?! (Let's say it's even $15 per tank refill, shall we?)
Cost per 100,000BTU: $3.36
Per my local utility company, NG is $0.803 per therm
1 therm = 100,000BTU = $0.803
Cost of NG vs tank of propane: $3.58 vs $20 tank refill!
NG is approximately 1/4 the cost of propane. Holy jeebus! Propane is expensive! Look who's glad I got that conversion kit, now, huh..? THIS GUY!
In short, if you're thinking about using NG instead of propane, DO IT! You'll save in the long run. I should make that extra $15 I spent on the conversion kit back in about one tank of LP!