I had the same thoughts back in January 1999, and the price differential between LPG vs Petrol/Diesel wasn't so great. Back then I was paying about 36p a litre for Autogas, and the conversion cost was £1200 for twin underslung tanks in March 1999.
67,000 miles later I'm getting an average of 19mpg on Autogas, driving steady (never over 60mph on motorways - sorry for being a tightwad on cruise control for any queues forming when I pass the trucks slowly).
Just had the tank straps replaced, the originals had rusted badly, these new one's (two per tank) use quite a lot of metal, so must be many times stronger. I understand that three straps per tank need to be fitted these days with updated regulations.
In 1999 there were only 300 filling points for LPG in the Country, today you can virtually get it anywhere, but I can get mine cheap 24/7 (see my signature)
That should be enough for you to make a cost benefit analysis, based on how long you intend to keep it. I'll be hanging onto mine until it costs more to repair than run it.