according to shell's FAQ, the gas consumption will be 10% to 15% higher due to the lower energy content of LPG. also according to shell, it's 25% cheaper (if gasoline reaches 40 pesos per liter, autogas should be 30 per liter).
since i'm consuming around 80L/month of gasoline (3200 * 40/L), that would be ~90L of autogas (2700), a savings of 500 pesos a month. i don't know how much the conversion costs but shell claims, 18000 pesos. So it would take roughly 3 years to recover the cost.

at least, cleaner-burning, less engine wear, etc.