I priced it up a couple of years ago, £3,000 for a V8 (2 units). I would have to do 25,000 miles on LPG ALONE, to break even. Mileage only 80% of petrol.
LPG very scarce in Spain & Portugal. To get a large tank (500/600 miles range), takes up a lot of storage space unless you can get it underneath the vehicle. Not sure they are a good thing either as engine designed for regular petrol.
As only do 2,500 miles a year didn't bother.
If you buy an RV with one installed it may be a different matter.