Depends how you do, Pete B and I normally stay at a Premierinn at Ashford North on the Friday night rather than getting up mega early, so thats usually £35 each from memory + food & beer, tunnel tickets normally work out about £74 depending on how you buy them (i Know Clem uses Tesco Clubcard points), then its fuel cost to Ashford and in Europe. From the midlands I would suggest 2 and a bit tanks.
Then beer and food on the Saturday night and Sunday daytime, (Pete B tends to do a communal curry which we chip in for so no food on Saturday night as such)
Hope that helps??