Is it an option to quote a package for 1 week instead for all the guests and you and your hubby can stay 2 weeks? It's difficult to tell people what to do with their money and how they should save as that puts them on the defense. We are also planning a destination wedding and travelling once a year somewhere tropical is no big deal to us as we save up in advance and go away each year. When we'll hand out invitations we'll have to be conscious of the fact that not all our friends and family travel the way we do and even a one-week vacation somewhere out of the country is not regular routine for some. I would love for everyone to be able to take the time off and save up the money to join us, but we are Ok with the fact that it may not be possible. We are going into this comfortable with the idea that it may just be the 2 of us on a beach getting married, and that's completely fine.
Just tell your friend that you would love to have her there even if it's less than the time everyone stays, and invite her to all the pre-wedding events. It's her choice to participate in whatever event she can afford to and would like to be a part of. Whenever she starts complaining about costs, just say "I'm sorry you won't be able to make it, we'll try to catch up when I'm back" and change the subject. Don't let her drag you down into a wormhole of self doubt and wedding guilt.