I just know I will have this problem down the road. My girlfriends have always been super flakey making plans but always backing out. Why does it seem the guys can make this happen better than the women?!? ugh I think everyones advise is great. You need to sit them down and tell them all your concerns. Then add in some language about finances, how you understand, just need to give a concrete yes or no so that you can firm up your plans. Unfortuently people are really reluctant to talk about finances so you will have to be the one that brings it up. I'm sure after you address it head on you and your friends will all be relived!