Before they start earning stuff from crime, Jane seems happy to indulge in Dick's fantasy of robbing a place, while not thinking he would actually go through with it. Wouldn't she care that they were using up petrol in the car?
Well they had surely sold their bikes at that point (although it could have added some humor, eh? =P). Also keep in mind this took place in ~2000 before the dramatic gas price hikes so it wouldn't be a huge concern. Then again they were doing chemical tests for a whopping $14 so who knows. It's a movie and a comedy at that - stop analyzing and enjoy.
While we are at it, where did the money for all the costumes come from? They were funny for sure, but to spend money on costumes was not necessary. I know, shut up and enjoy the film...........
Over here we roll our eyes sarcastically when people say gas instead. Gas already has a meaning, so why double it up and not just use petrol? Or even fuel if you're that desperate to not use sensible words.
- You must perform the ritual... in a tutu. Pillock.
I'm assuming that "petrol" is short for "petroleum" which also already has meaning in the phrase "petroleum jelly"...
And one person's "sensible" is another person's "whatever works for you." "Petrol" works for you, "gas" works for us. 'Sides, if we used all the same words as you guys, you couldn't tell us how we're butchering the language. :)
Like ajstars1183 said, in 2000 gas only cost ~$1.25/gallon, which is about 31 cents/litre if you live in Europe. The car they were driving was very small and probably got very good gas mileage, I bet they could have drove around for two weeks on the $14 dollars she got. About the costumes, if you notice, they didn't start using costumes until after their first successfull robbery. That's where the money came from to buy them