It would be £150 if me, my dad and a mate of mine went, plus food, petrol etc.. looking at £200+. Probably worth it though.
I would say it's totally worth it for you, of all people!
You can get up close and personal with some awesome machines and drivers.
I've been 4 or 5 times, can only get away with a one day visit nowadays, but for a true petrolhead you should do the full three days (four if you include the moving motor show).
Price wise, you can go silly or sensible. A three day ticket, camping and BYO packed lunch and you'll save yourself a fortune compared to buying food and drinks there, hotels, etc.
Plus if you camp you'll be with a load of like-minded individuals and it will have a kind of festival feel to the campsite.