I was actually just roughly calculating this out because I'm brewing with a friend tomorrow and dont want to let him use all my propane for free.
With a 10 gallon batch and a 55K burner I calculated it out to approximately 5lb of propane used with the burner on full blast including heating HLT water, 90 min boil, and water heat up from mash tun
Even more roughtly, if you take the BTU of the burner in hours (in my case 55000) x figure out how many hours you are heating or maintaining boil for, and then divide by 22,000 (22000 BTU per lb propane) youll get pretty close to lbs of propane used