You've got to do some serious miles to get the install cost, or kit cost to pay for itself.
The cost difference is not as great as it used to be, then add around 10-15% less economy per litre, but oil lasts lot longer.
So I suppose depends on mileage, can take cat off and never have to worry about emissions anymore though, my last car was 0.0015% co, plugs last longer too.
Diy is no prob if competent, wet kit works much better, really need a spark advance unit to get the best.
My dad had his old vec b x20xev converted to LPG so he didnt have to pay for the conjestion charge going to work each day, so that helped pay for the conversion. People said to him that there would be a small difference in power and economy. But he ran without a spare wheel and only a tenner max in the petrol tank and it was still on level pegging with the petrol power and economy. It would still keep up with most rep mobiles in the outside lane, or switch it petrol and it used to take most things.
If your doing the mileage to warrant the conversion, id go for it. But down this way it used to be tricky getting fuel at the same pump often had to put more petrol in to get to the next LPG station.