1 mile of elevated roadway costs 20-30 Million. Grade level Bituminous concrete on a sand bed....Not so much.
So the current projected cost to replace the Bonner is about $215,000,000.
The distance between the south end of the Bonner and the Chicamacomico Life Saving Station is 13 miles. Even using the middle of your estimate at $25,000,000 per mile of causeway, the total cost of the project would be $540,000,000. That is still about a half of what the 19 mile long bridge would cost. I would believe that maintaining a land based causeway would be less than one completely over water.