I got married on a private beach in St. Thomas for under $4k. through Royal Romance on Royal Caribbean. We sailed on the Freedom and they took care of EVERYTHING...transport, marriage license, pre-cruise questions etc.
The only thing logistically would be transport. for the 50+ you would have coming. Otherwise, it was a BEAUTIFUL ceremony on a gorgeous day. Pictures are still the envy of our friends to this day, especially the one of me holding my wife up along the shoreline with turquoise water behind, white sand below, and huge smiles on both our faces.