My second truck, a 95 F250, was converted to LPF shortly after I bought it.  At the time propane was 16.6 cents per litre and gas was about 70.  I put 247k on that truck in 44 months.  Running the propane gave me a relative fuel economy of 68 mpg.  I only drive about 20k per year now so the conversion would not be worth it.  Even if that price differential still existed.