Of course it depends majorly on whether your do counter service or sit down restaurants, in the parks to eat or offsite, clothes shopping or not.
When myself(20) and my BF(24) go, as far as we're concerned we're on holiday and although we're not stupid with money we don't penny pinch in the slightest (not like back at home!) We also have all our passes before we go and we do a lot of shopping. I don't like wasting time umming and ahhing while on holiday so if i see something i just buy it (again not like back at home!). Therefore, we normally take about £2000. We have more available to us on credit cards if i go a bit mad, but we don't need it.
I'm sure you could do it for less, but the way i see it, we don't go every year and if it takes us 1 and a half years to save up and pay for it all, so we might as well go all out when we're there.