I stayed on board at this port as I have been to Barcelona a few times. The rest of my group paid EUR8 each to get the shuttle bus to and from the town and I think they all had a good time. The shuttle bus ran every few minutes it seemed.
EUR2 taxi in to town or a fairly short/flat walk. Here they take sterling and it is like being on any British high street. We didn't go up the rock as we thought it was a bit expensive for what it was.