Money is to pay for things on your trip, but spending money should not be the main reason for the trip. It is used to get you where you want to go and to acquire what you need, but it should not be the main thing that you think about on the trip.
In the same way, you would not continually look at the gasoline level in your car and spend a lot of time trying to figure which gasoline to use.
Both gasoline and money are used to get you somewhere and serve your basic needs. Neither of them should be the one factor in deciding how much fun you have.
Addition: Idunn makes a very good point that the sentence could be about money spent during a trip or money as it relates to life in general.
The sentence could be interpreted:
Money in life is like gasoline during a trip.
However, the sentence could be understood either way, given the way it is worded.
In the end, understand: Money isn't everything.