let's take the worst case scenario..

an 8th gen civic can give you 8/100k (in facts it can vary if you do the city driving, you can get up to 10/100k)
while the lpg-fitted car can give you 15/100k

so

if you need to do 100k, the civic will cost you:
the civic will cost you: 8 litres x $1.25 = $10
while the lpg-fitted car will cost you: 15 litres x $0.50 = $7.50

Conclusion:
there is NO way you can beat the lpg...only toyota prius with its hybrid machine can match the lpg so far.