Getting out of plans with a friend

This is going to sound terrible. My friend asked my to go on a trip with her this summer. I told her I'd have to think about it. So eventually I said "let's start making plans so I can see if I can afford it." She started telling me how excited she was and how I couldn't bail out on her. Now. I was her 3rd choice. Her girlfriend can't go because she won't be 18 by the time the trip happens, and her other friend wouldn't go (she has told me 2 different reasons why she wouldn't go so who knows what the real reason is). I don't want to go. I need to save money to get my apartment and pay for college... she gets everything handed to her, so she doesn't understand that. (By handed to her I mean her grandma bought her a brand new 2015 car and GAVE IT to her.) She wants to get the hotel illegally under someone else's name (because she refuses to look at hotels that allow 18 year olds). That's reason #1. Reason 2 is that I just overcame an alcohol addiction, and the point of "senior week" is to drink. She says she won't drink, but she bitched at me on new years for not drinking with her. Here's the question: how do I tell her no in the nicest way possible without breaking her heart? I find it impossible to be mean but I don't even have a job right now to save money from :(