To convert your car to use natural gas, you would have to buy a natural-gas engine. These cost about 8k- 10k. You could recoup some of the money by selling your old engine, but it is still a steep upfront cost.
Here's an article if you're interested in reading more: http://www.theatlanticwire.com/national/2013/05/why-youre-no...
The original article states "Kits to do so cost anything from $12,000 to $18,000" which feels quite strange, as in Eastern Europe the major car service companies offer the kits for at most $1,200-1,500 with installation. Sure, the car mechanics earn a bit more in USA, but it's not really that much labor and the actual hardware should be cheaper in USA - all other hardware is, due to larger scale and smaller taxes.
Also, I think you'll find that the cost of things in Eastern Europe is commensurate with their earnings. You can't compare the cost of things there with the cost of things in America, just like you can't compare the cost of a home in India vs. the cost of a home in San Fransisco.
But cost of things nowadays is not commensurate with earnings, and you definitely can compare cost of things (not food, services or homes, though).
Stuff is produced globally, consumed globally and priced globally - the same globalized physical item, regardless if it's a Honda Civic engine, an iPad or a commodity part #12345AB, can be easily compared across the world - it's all made in the same place anyway, shipping costs are comparably tiny, and the USA will usually be the second cheapest place to buy it, right after the actual country of origin; due to economy of scale and tax differences. If I can buy a physical global-commodity nonperishable item for $x, then it is almost always available in USA for 80%-90% of that, I've not seen any significant counterexamples other than custom hand-made stuff.
The wholesale price of a pallet of LPG conversion kits as coming from the factory in China or Indonesia would be nearly the same for USA, Somalia, Switzerland or Argentina - and there's not enough labor involved to justify a $10.000 markup on that unless $500/h lawyers would be doing the installation.
Well, that's a new once, since everything I've ever heard is the exact opposite.
I know propane conversions run forever.
Suggests there is something to worry about in a conversion for regular cars.
Let's say it costs $3,000 to have things converted. This is about the cost for an LPG conversion in the UK, though that might be an underestimate, as while LPG requires a bunch of stuff installing the engine still works in fundamentally the same way. How many gallons (x) will you have to buy before you've spent the same in both cases, and what will the cost have been (y)?
Assume $4/gal petrol and $2.50/"gal" gas.
4.00x = y (petrol)
3000 + 2.50x = y (gas)
So, 4x+-y=0, 2.5x+-y=-3000; 4x+-y=0, 4x+-1.6y=-4800; -0.6y=-4800; y=8000. So x=2000.At 20mpg, 2000*20=40,000 miles. That's quite a lot of driving. Don't most people change car every 3 years? And by that point you've only just broken even.
If you have a Honda you can easily reprogram the ECU to compensate for running a different type of fuel (you would simple need to replace the fuel system, which means just a high pressure tank and a regulator) I could see this working for ~$1500.
On other cars you might need a piggyback computer. (source: I converted my car to ethanol which has a few similarities)