As Westle said, weekends are expensive.
If you're on a budget, this is how it breaks down. This is about as cheap as you can possibly do it:
Ferry £70
Fuel £100 each way, so £200 (assuming a few hours to Dover from whereever you live)
twin room £75 per night, so £150.
Those are your shared costs, £420 total for travel and accomodation, shared between two people £210 each.
Then you've got food, assume maybe £30 per night for a meal and beers, that's £60, plus maybe £20 for two lunches, that's £80. you're now at£290 each, for a driver and passenger.
Then, as a driver, you'll need laps:
8 lap ticket £140, 3 laps saturday afternoon and 5 Sunday would be more than enough on a first trip.
That'd put the total cost in at £430 for a driver, and £290 for his passenger.
I think it's fairly safe to assume that between 2 of you in a car the total cost will be around £700. I went out there for a week in september and spent less than £500, and did about 16 laps as other people paid as they wanted the ride. Yet I went out last easter, and had a pretty hateful car failure, and it ended up costing me well over £700 for a 3 day weekend trip!
It's ****** worthwhile though!