here in the uk conversion can be anywhere between £1500-1800. lpg costs 1/2 price of the petrol and the fuel consumption stays the same. taking an average cost of the petrol £0.90/litre and cosidering 21 mpg (combined) for the ML320 means its well worth installing the system when you drive at least 10K miles per year (right for the uk prices on gas).
here is the math.
1. 70l full tank costs here £63/ea
2. it is enough for around 388 miles
3. lets say you make 10K miles a year which will normally cost you 26 fillups (£1,640.00)
4. if you invest into lpg it will bring your cost down to £840 per year since the price of lpg is half price of the petrol.
5. meaning after 2 years of at least 10K miles each you will starting saving your money.
i myself made around 18K in the first year of owning an ML so if i installed it in the first place, it would start working for me already now.