With deisel, you have to pay a lot more up front for the car, the engine is more complex and potentially has more to go wrong (higher pressure) and diesel is roughly twice the price as LPG.
I calculate that car purchase cost aside it's still roughly 15-20% cheaper on LPG than on diesel.
I bought my 330ci for less than £1500, if you can find me a good 330d for less than £4000 I may look. I used to have a Mercedes C270CDi so have a rough guide.
At the end of the day, it's also the thought of the conversion that makes me interested, the saving of money is a big bonus. If all I wanted to do was save money then I'd buy a 1.2l VW Polo or something else equally as dull but if you drive a long distance every day then having a car that is more fun, interesting and comfortable is just as important as MPG.