Ive never driven at gatebil so cant comment.

We normally go for about 5 days, so thats skewing my costs a bit I guess as to me I still class that as "a long weekend" which I guess most people would say its more than?

Basics though with real vague figures:
Tunnel crossing + fuel to get over and back : 200
Accomodation for two : 400
Food for two for 5 days : 250
Breakdown cover : 100
Petrol and laps : 600

Thats 1550, and then you have some wear and tear on the car, tyres and brakepads etc.

So yeah, 1500 was a bit low if anything and thats before you start going to the museum or going offroading and stuff too.