Main cost is going to be petrol and lap tickets. Ferry will be £50-110. Hotel and food depends on you really. There are loads of guesthouses or you can even camp if you want to save money.
Opening times can be found on various websites like this one:
http://bridgetogantry.com/index.php?option=com_events&Itemid=72 However, they are well known for changing the dates of public sessions without any notice. Beyond that, just turn up, buy a ticket, enjoy.
Ben Lovejoys site is a good read.
I shared costs with my brother when I went and it cost us about £400-500 each for a couple of days. We didn't try to save costs... We then had about £1k worth of maintenance to his car when he got back! He went recently, drove a lot harder and his car required no maintenance when he got back. It's a hilly, bumpy, 13 mile punishing track so it can be hard on the car...
Personally, I would take a few days off work and go mid-week to try and get some quieter laps in rather than on a busy weekend where you may get loads of track closures. A couple of my friends were out there one weekend the other year and there were loads of track closures due to 4 fatalities...
The insurance situation is a bit of a sticky one. As it's a public toll road you are legally required to have insurance cover and the police may investigate major accidents.
However, as a UK driver you are going to struggle to get insurance cover... Most people just drive it uninsured but can you imagine what would happen if you killed a biker or hit a Porsche..
