We had our wedding in Maui. Yes, it could get expensive. We all found really good flight deals to get there and I made sure to let people know once we did find a good deal. Alot of our single guests stayed together in a condo to make it even cheaper.
Also we treated our guests that came to a bbq the night before the wedding, a reception dinner at Seawatch the day of the wedding, and a luau the following evening in Lahaina. We wanted our guests to know how much we appreciated them coming to see us get married. Plus they got really awesome OOT bags. Granted we had only about 20 people there total, but I think those things we did for them made it a bit more reasonable for people to come. Alot of them made a great vacation out of it.
I think if you are able to be on top of deals for your guests and let them know as soon as you see one, it can make it quite an affordable vacation for someone. OR get a TA to do all that for you
Good luck and don't feel too guilty. The people that want to be there to see you get married will be there