Mine was converted 3 years ago professionally .. at a cost of £1850 + VAT ( VAT was 17.5%) = £2173.75
Tank difference in prices was small really, system was a major cost (I went BRC) and most places quoted big differences in 4 pot v V6.
Took just shy of 2 years to "pay for itself" as the saving is around 10p per mile - doesn't seem to change much over the longer term as LPG /petrolprices rise and fall, although in the short term can drop to around 8.5 - 9 p per mile.
I researched LOTS and if I'm perfectly honest .. I believe the general consensus is that mine is possibly the only decent professional install around .. most others have serious problems.
If I were forced to change the car I would go LPG without a doubt... but would use the collective knowledge and skill of the forum members to assist me with a "DIY" install .... given sufficient "persuasion" (in liquid form ...

I think its possible to get them to do a great deal of the skilled work .. leaving me just to "hump and dump"
Not that I intend to change anytime soon !!
HTH