I dont see anything wrong with that.. Its your decision as its your money.. I knew once I moved out that was it.. If I chose to go on vacation with family then I was responsible for it.. Now if you are inviting her to go, then perhaps you should pay.. but if she is going per her idea, then if it were me, I would say its on her.. There has to be a point at which you have to say "if you want to be considered an adult, than you have to take what comes with that" .. You cant pay for your children forever..
We are taking a disneycruise in 2008 and it is our family vacation.. DF's daughter will be 20 then and will be invited and paid for as will be her boyfriend because we are taking DGD.. Since she will be 2, I dont feel we should take her without her mom and dad, since we are inviting them we will pay.. but that may be the only trip we can do that.. There will be 9 of us there which will be extremely expensive and I dont foresee us being able to afford it more than this once.. I feel like its a one time thing.. After that they will have to pay for some if they chose to go..