I think that a lot of the brides on here "take it" from their families. We made the same decision after we added up the cost of a big wedding at home. I am lucky to have such a supportive family. After they got over the inital shock, they were happy (even excited) about it. They are all talking about how they are going to loose weight so they can look great at our wedding.
Your mom might not ever come around. My mother is very traditional, and while she accepts our decision, she always ask, "Don't you think you will miss not having a church wedding?". And I can truthfully say "no". As long as you are okay with what you are "giving up" in order to have a destination wedding, then nothing else matters. It is about you and your FI. Don't focus on the day, focus on the life long journey that is being kicked off by an awsome vacation!