So, basically, rip off everything and start with the inner pressure vessel from a Dragon. Might as well do it right and just clean sheet it.
Not really. Of course that a Moon lander based on the Dragon will be a completely new vehicle. That goes without saying. I was contemplating the Dragon landing on the Moon as-is because it's good food for thought.
However, the know-how is there, and SpaceX have already done their homework in building such things. Designing a new moon lander for them will be significantly less difficult than building a capsule from scratch.
Guidance systems – done.
Major engineering hurdles – solved.
Propulsion – only little rework required.
Interior – I'm pretty sure that they'll nearly CC it.
I'm not sure about the figures, but if I remember correctly, designing Dragon costed $300-400M*, building one vehicle costs $100M, and designing the SuperDraco cost a couple hundred million at most. My bet is that the moon lander design will be in the SuperDraco price range. At worst.
I'm also willing to speculate that expansion to landing on Solar system bodies has been considered in the design and it might require a lot less work than we're thinking, but maybe I'm giving them too much credit here.
* Or was it $600M? I'm not sure if $600-700M was the price for the Dragon and Falcon 9 combined or for each one of those. I don't remember in which interview Elon said that to check.