Impossible to say really. It depends on how much of a big spender you are.
I've been a few times, and took about a �100-�150, but I didn't spend it all, though there are people who could probably spend well over that. As well as clothes, there's lots of hair and beauty stuff. It's good for stocking up on perfume, because it tends to be cheaper there. You could probably get some christmas pressies there aswell, depending on what kind of things you like to buy for people.
You'll need money for food and drinks, unless you're bringing it with you. I can't remember but I think you have to pay for parking.
Top tip: Wear flat shoes. It's quite gruelling walking around all those stalls!
Give Dermot a wave for me if he's presenting the catwalk show this year. Hope you have fun.