went on my 7th trip two weekends ago, I went in TDi golf, so quite good on fuel but you still need to budget approx £600 Id say for everything, crossing, b&b, fuel, food, laps, and dont forget a little more in case you have any mishaps. Always get breakdown cover as well.
Bearing in mind I done around £700 this past trip but I did do 12 laps at a cost of 285 euro and went out for Steak 3 nights running at 25 euro a night, and took the train, which is a tiny bit more expensive than the boat, but so much quicker you can get to the ring the same day you travel and have the afternoon there.
We always stay at Sliders Guest House, 15 mins from the ring, but a nice drive there and back, and its only 32 euro per night, beers fridge always full for the guests. Spot On.
Im back again late august early september, and planning on moving over there sometime next year hopefully, i love the place/atmosphere