As above, it all depends on your mileage, but as a very rough guide, if you're spending £200 a month on fuel, therefore £2400 a year, then you'll spend about £1200 instead, after conversion to gas.
So you'll have covered a £1200 conversion cost, and from then on, you're halving your fuel costs.
I say roughly, because that doesn't take in to account the cost of petrol for a couple of miles when starting from cold, or the slightly worse mpg on gas compared to petrol, or servicing costs for the lpg system.