long weekend can be done for £600 per car if you budget right, and dont do too many laps, im off again end of june and will be taking £300 with me and expect not to exceed this by much. lidl for food during the day, pinocchios pizza at night and 2 laps a day, then some sightseeing around the circuit the rest of the time.
before i get knocked down for this estimate on costs like last time, ive been going for 5 years now and have still yet to exceed £400 for 3 days away (including ferry, fuel, laps, food and accomodation).
depends how crazy you want to go, its VERY easy to spend more if you have more.