As for the LPG conversion - I don't know the price relation between LPG/Gas in US, but here in Poland LPG costs half the price of the Gas.
I had LPG installed in my daily driver - Mistubishi Lancer... no, not EVO - 1.6L sedan - after first 1000 km (i bought it new). Now, after few years and 110.000 km I estimate the money saved at c.a. 6-6,5K USD. And it's fuel consumption is ca 10 liters per 100km. The higher the consumption the higher the saving .
No issues with the car, however I've heard that some engines are not recommended for using LPG. The temperature of burning the LPG is higher than Gasoline.
You don't need to remove gasoline system in either carburator nor injection systems - they will work on one engine. The system uses gasoline when the car is cold, after reaching "operating" temp. it switches to LPG. You can manually switch between them as well.
One of the cons of that solution is the range of the car on LPG - the tank that fits in spare tire place have the capacity of ca 35 liters - you can use bigger tanks, unfortunately they limit the space in your trunk.
The main problem You could have in US is finding the LPG fuel station, where You'd be able to refill...