I'd say you need to decide to go or not. I don't think it's inappropriate to have a discussion beforehand about where to go and being cost conscious. I DO think it'd be inappropriate to say anything about "the aunt" paying for her family. Rather you could say something like, we'd love to have some input on where we go, stay and eat as we really love going on vacations with the whole family, however we're also trying to be cost conscious.
If the selected destination, hotel, etc is beyond your budget, then politely decline to go. If anyone asks why, just say that it's out of your budget, but you wish everyone else a great time and can't wait to see pictures.
This is how things were handled with our large group of friends...as families grew, jobs changed, etc it became harder and harder to please everyone. It seemed like the best choice was to have one family be the "planners" each year, that way everyone got to have a final say on where to go, make the arrangements, etc. If one family didn't want to go, couldn't afford it or had other obligations, they would simply decline before any arrangements were finalized.
Perhaps these ideas will help you.
Best wishes.