i totally felt/sometimes feel the same way. a lot of our friends are just out of school, and our families are fairly moderate. but in the end, we chose the dw because it was easier for us to keep it small, and my fiancee & i ended up paying about 5X less than we would have if we'd stayed in town. our family & friends seemed to recognize this when we explained our reasoning. we also let our guests that they had the option of to just staying at the resort the night we're getting married (at the riu have to have that 80% ) if that would be more affordable for them.
that being said, when we sent out our save the dates, we stated that we knew not everyone who wanted to come would be able to make it, but we'd be hosting an informal reception upon our return with pictures, food, etc. people seemed to be really excited by this.
best of luck!