It's possible they won't enable the software for another hitch as aftermarket hitches are explicitly prohibited in the manual.Tesla charges $1250 retail, but that includes the Bosal hitch. I think you'd have to call the service center to find out how much to enable the software if you're installing your own hitch. If I had to guess: $750. The software is enabled through the service port under the display, so it shouldn't take long.
That said, people who have the Bosal hitch but not the tow package (early deliveries) are paying $750, as you mention.