Well, as a new member who was faced with much the same dilemma, if it helps any, I can offer a perspective after recently buying a converted RX300 (albeit an older imported Harrier). My experience so far - based on being an LPG newbie and having a pretty rare 2.2 model - is that you need to factor in more than just aspects of fuel saving.
I've no doubt there are far more knowledgeable members (and qualified technicians) than I, but I hope that my view is fair and reasonably objective.
So, having been camped out on the LOC site for about a month and on pretty much every LPG-related forum, installer site and those in officialdom, together with lots of phone conversations and pints being bought for people, you can tell I take my research seriously :)
Much of your choice will depend on whether you want to get your hands dirty or would prefer to be a chequebook motorist (as they used to say before Chip & Pin) or somewhere in between.
Assuming you want to buy an RX that has already been converted (which means someone else has already stumped up the install costs) run the number plate through the UKLPG site to see if it is registered. That will tell you if the system was installed/certified in the UK. Mine wasn't as it was done in Poland. An overseas install may be fine (with a cert from the specific country's jobsworth's) - usually done for a fraction of the UK cost - or it may be dodgy.
The next step - apart from knowing where the local/usual route planning autogas pumps are - is to locate a local approved installer who undertakes servicing. Most if not all systems should be serviced annually. Bear in mind that there at least a dozen different system manufacturers. A bit of headache for me was finding out that the nearest approved guys don't have the software plug for my system. This means that you may have to go further afield every time something needs checking and possibly at greater costs. Servicing seems to be about the £150 - £300 mark, depending on how many cylinders/cc you have and type of system (single point, multi-point, sequential, etc.) and how long it has been since last serviced.
Another important factor is that your insurance premiums will likely jump by some margin, compared with a non-LPG vehicle. Most want evidence of your car being on the UK register, but not all do.
Okay, in terms of actual day-to-day running costs, bear in mind that the car should start on petrol and be warmed up before switching to LPG (a separate gauge/switchgear somewhere on your dash). So, you need to keep at least a 1/4 tank of petrol in the tank as standby (and maybe a carry can in the boot if you're a nervous type). And you may need a valve lubricant - £2-£10 depending on branding, but may be needed to stop your valves burning out due to the cleaner fuel not containing the sort of lubricants that come with petrol/diesel.
Bear in mind that there is also the cost of plugs and plug leads - they take quite a bit more hammering in higher temperatures than in non-LPG cars. Make sure the plug gap is set right (or, remind your garage to use the same type of plugs as the ones that came with your car, assuming it was running right in the first place).
If you're a nervous type, this next bit won't fill with you confidence. I can confirm the assertions found hither and yon that there is absolutely no point relying on your petrol gauge/LPG gauge. After an LPG install, they become controlled by unknown forces, behaving as if you've just entered the Bermuda Triangle. Some say that one or the other or both reset after driving 150 miles. Some say otherwise. Some say relax, forget it, they are now redundant bits of kit. Either deliberately fill both tanks and run them dry (not a good idea, really) to find out how accurate they are, or, learn to fly by your tripmeter(s). On the plus side, it harks back to yesteryear, before driver aids, when it was down to skill, terror and the driver gods moods...ah the open road...so much more fun. Oh...and you better renew your roadside assistance membership....another hidden cost :) You'll find a bunch of folk on here querying actual real world mpg figures. If you're opting for the 3.0 V6, most seem to get about 18mpg in mixed driving. I do better than that - a theoretical 25-28mpg with mine (2.2). It's only still on the first tankloads, so yet to be confirmed. I am hugely entertained by the hilarious 'cruise information readout which challenges my maths: average 4.4km/l. At first I nearly had a heart attack when seeing those figures but by all accounts, forget the computer, as it is still under the impression that you're using petrol in one tank. My tripmeters prove that hopelessly (fortunately) wrong - but again, depends on your driving style, route planning, payload and, as some clever wag said....what colour underwear...you get the drift...
You might also find this link useful: http://www.go-lpg.co.uk/hints.html
Overall, the following analogy might help your decision. If you run a business, you know that there is a big difference between gross profit and net profit. When people talk about the fuel savings of switching to LPG, it correlates with gross profit. Much of the stuff I mention are the overheads, not always fixed costs....but after that, you can consider a true net profit as well worth the effort.
I'm glad that I bought an LPG converted vehicle. Even more delighted that it is the RX300. Sort of. Well, yeah, it IS :)
I await your decision with interest: perhaps we'll see you around here telling us all about it! Good luck!
PS don't worry about quoting this post in any reply: a long one and hope I haven't upset the natives...