The economics of having a car converted really depends on the condition of the car in the first place, and your intended length of ownership.
If you've found a nice car and intend to keep it for (say) 5 years, then spending a couple of thousand on gas is quite practicable, especially if you cover sufficient annual mileage to recover the costs involved.
You will get some of the money back on resale anyway, so you're not looking at recovering all of the outlay.
Don't forget that at 12-16mpg on petrol, you wouldn't be driving the V8 in the first place, it's only the half price of LPG fuel that makes it practical for most owners.
Shop around for a conversion, I think you could get a multipoint system for just under £2k these days, possibly less as the installers aren't exactly busy right now.
Peter