How do I let my friend know that I can't afford to go out all the time?

My friend is a bit more financially stable than I am, and really wants me to go with her to a concert in the Fall, the thing is the concert is a plane ride away. She already bought hotel reservations, but I really don't think I can afford to go ( I still need to come up with plane fare and miscellaneous expenses) I love her dearly, but I don't want her to feel like I am always making up excuses not to hang out with her when I just can't afford to spend the money. :( She is always treating me out but I would prefer to just not go out. I treat her back when I can. But I don't want to seem like a leach. Especially when I don't really travel or care to do expensive things. Please help! thanks.