I Checked my Titan 575, the first £35 of petrol ran the machine for 8.5 hours, if it had been on lpg it would be half.
The 575 is a 32hp Kohler.
For the £35 of fuel it earned about £1500 so costs are around 2.5%
The next tank of fuel earned slightly more, so all in all costs are low.
Shaun Ashmore has same machine and his costs will be much less as he charges more and his runs on lpg, you will have the cost of lpg installation back in less than 6 months normally.
I think it was Simon Gerrard that told me the saving in lpg on his Titan paid for the machine !