Been looking at this with interest. Here's my take as someone who has run several LPG motors and converted even more.
LPG generally reduces the fuel costs to 2/3 of running on petrol. This takes into account the petrol for starting, flashlube, etc.
To give a fair comparison of the 2 fuels you need to compare the cost per mile, nothing else is accurate. This is how I have managed to get to the saving figure. When I first started running on LPG it saved about half (LPG was 30p/L and petrol was about 80p/L) but with the price increases it is now more like a third. This has remained steady now for about 3 years.
So, taking the rather odd example used above. If the petrol running costs are £120 per month for 3 years, LPG running costs will be £80 per month for 3 years. This means a saving of £40/month. £40 * 36 months = £1440. A DIY kit for a 6 cylinder car can be fitted, inspected and registered for sub £1k (last 6 cyl kit cost me £845 all in) so take that £1000 outlay away from the £1440 savings and you have saved £440 straight away.
Additionally, it burns much more cleanly so is more friendly to the engine and also qualifies for a (small) reduction in Road Fund License for post 2001 vehicles of £10 per Year. So there's another £30 making the savings £470 immediately Yes, there is a little extra cost in general maintenance (filters) but the clean burning of LPG will mean that the engine oil maintains it's properties longer. So instead of changing oil every 4k max (as I tend to on petrol) it can be extended to 7-8k and still come out a nice golden colour.
Moving on to the other things raised... Modern (Fully Sequential) LPG systems can easilly perform exactly the same as on petrol if they are correctly fitted and tuned. This is fact! Yes, with the old mixer systems there was a loss of power but even then they could be tuned to minimise it. But that is old technology and things have moved on considerably (as they have with cars as well!)
The number of LPG filling stations is increasing year on year and in the last 5 years I've only had to run on petrol (because of running out of LPG and not being able to find somewhere to fill up) for a maximum of 500 miles of my 100k+ miles driven.
Finally... Whilst I accept that LPG isn't for everyone and if your petrol bill is only £120/month then it's quite affordable. With the distances I drive my petrol bills were more like £150 per week so LPG was a no brainer and had paid for itself within a few months. That car did 77k on LPG, saving 9p per mile, which cost me £800 to convert. 77000*£0.09= £6930 in savings. Take £800 off of that £6930 and I saved over £6k in fuel costs running that car! That's not to be sniffed at ;)
What I would say is that LPG is a no brainer if you do enough miles, regardless of what anyone elses rather strange calculations show ^_^