A fully sequential system like the BRC Sequent system I have will set you back anything from £1300-£1700 inc depending on who fits it, as dealers vary. I had the benefit of my company paying 2/3 of the conversion costs, so it was a no-brainer for me and my local Autogas fitting place had a deal on for £1149+VAT.
I can't notice the difference in performance between LPG and petrol on my system to be honest and whatever it is I'm not bothered by it. The car is slower than standard but I have a 55L Stako tank in the rear, which will probably weight the best part of 60kgs or so when fully filled. Incidentally a whilteline rear anti-roll bar really helps the car in the corners. Standard it really was horrid on corners with all the extra weight.
Fuel economy from my setup is about 10% less then on petrol, so ranges from around 36mpg to 44mpg on LPG depending on how I drive, (equates to between 340 to 420miles to a full tank), and at circa £21 to fill, them be cheap miles!
It all depends on your budget really, at any rate prices from 47.9-53.9 per litre is a pretty good motivator for those who do plenty of mileage in the current market. Frank did a DIY system which had been going for years IIRC. A single point system, although not technically as good as a sequential system, will still offer very good returns, (mileage dependant), and is very good value for money. Saying that the sequential system does fit a K11 very well since it's already sequential to begin with and compliments the LPG system very well, (all be it more expensive).
At the end of the day it's your choice
