Here in Belgium LPG is around €0.35-0.50, unleaded 95 is around €1.20-1.50 depending on the brand and location.
With my driving habits my Doblo consumes either 8.5-9l/100km petrol (about €12) or 11.5l/100km lpg (about €5).
Since my LPG installation cost about €2000, this means I won the investment back in less than 30k km on LPG, or about 35k km real world driving since the first few minutes of every trip is on petrol and I sometimes do a trip on petrol to flush the system and avoid the petrol in the system to go stale.
I've also done the comparison to diesel, as a Doblo T-Jet with LPG costs more than a 1.6 MultiJet, but it's still cheaper to go with T-Jet + LPG after about 80-100k km, which still is only 4 years for me.
Apart from the calculations, it just feels good to fill up for €20-25 and know it'll last me over 500km. That feeling is every time you fill up the tank, the investment for the LPG installation is only once and forgotten very fast.