I have done this by road a few times which has had to be at selected times as it is carried out by a police convoy. usually 8am, lunchtime, 4pm. Not sure if these are still the times but are approx. Costs vary depending on how u do it through an agent it will cost around £80 return. If you arrange it whilst there probably much cheaper at around £40 return.

I read that one of the other members has used the local bus service which is less than £10 not sure how this works with regards to the convoy as being a foreigner!