How Pipe Natural Gas is Cheaper than LPG

Its because its the raw material for your LPG.

"The by-products of that processing include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, elemental sulfur, carbon dioxide, water vapor, and sometimes helium and nitrogen." Wekipidia

How come the raw material will be expensive than the processed by-product.

I'm not going to do the work for you, but the way I would do it is to calculate the BTUs for a given volume of both natural gas and LPG. From there I would find out the cost of that given volume to determine which provided the most BTUs for my dollar.

A lot of the cost factor is not just calorific value, but availability, transport costs, etc.

Where I live, natural gas is less expensive than LPG. Just off the top of my head, I would say that if you have access to piped in natural gas, that would be the most cost effective way to go.

calorific value (natural gas) < calorific value (LPG). Asuming natural gas is mostly methane, this is true on a volume basis, because density of methane is lower than LPG. On a mass basis, methane is higher (due to higher % hydrogen). Once you get past ethane C₂H₆, on a mass basis calorific value of hydrocarbons is pretty much constant irrespective of molecular weight.

Presumably you meant "nozzle hole dia (natural gas) > nozzle hole dia (LPG)".

As kramarat said, which is cheaper for you depends on local conditions.

Incidentally, liquid LPG is less dense than petrol (SG ~ 0.6 vs 0.8) so for an LPG car although it's cheaper per gallon, it's not as good a deal as you might think.

I once did a job based on a US spec (somewhere in north-east if I remember right) where fuel was described as natural gas but calorific value (volume) was about right for propane. Maybe higher MW natural gas occurs in US, anybody from there comment? Or is it just terminology?

Cheers...........Codey