I was not very impressed with the Salt Cathedral. It seemed to be one of those things made just for tourists. As for getting there, I used the hotel car/driver, which was probably the most expensive way of doing it, but it was comfortable and easy. It's far less than a day trip - a morning trip perhaps.

Aires had the best fares for the flights I took within Colombia. Their planes were comfortable, on time, and service was fine.
http://www.aires.aero/Home/ You should find roundtrips between Bogota and Medellin and/or Cartegena for about $100 US.