Our return trip to Florida, including driving while we're down there is usually around $200 US -- however we haven't driven since 2003 and the gas prices are a lot higher now. Does this person want to pay for your gas one way, or just her "share" (ie. there's already 3 of you going ... she makes 4 -- so she only wants to pay $25 of the actual cost of $100). You gotta figure that it is inconvenient to take along someone else, luggage takes up some of your room, another person to wait for when you stop for bathroom breaks, etc. I say if she offers to pay for gas every time you fill up, take her up on it. Remember, you're doing this person a favour ... not the other way around. And just because "you're going anyway" doesn't mean that you should only charge her something minimal like $25.
That's a tough one !
MaryLiz