You know how your friends are and that they are not going to change so if you really want to share this particular experience with them you are going to have to invite them as your guests and pay their way or just give up the idea entirely and either go alone or find new friends who can and will save to do things with you. It should be clear to you by now that these plans are a lot more important to you than they are to them and that your wishful thinking is just keeping you from having the trip of your dreams. It's understandable but your unwillingness to accept reality is only depriving you of a chance to expand your life. I'd just tell them that I'll be going on the trip at such and such a time and if they want to join they need to give you this amount of money by this date and that you hope they can do so because you really want to do finally do it with them. Then the day after the money deadline tell them you are sorry they decided not to go but will tell them all about it when you get back. Then go yourself or with someone else and have fun.