it all depends how long you are going for an what you what to do in that time.
a lad i work with has just got back from ibiza, he isnt a big clubber, he went to 2 big nights, stayed in bars the majority of nights, ate out every night and spent £450 in a week.
this year i am taking £1,500 for 2 weeks, and i am planning on eating out every night and going to about 8 big club nights.
there are ways of doing ibiza on the cheap, eating in your apartments, and the biggest money saver is buying your drinks from a supermarket and spening the majority of the night outside the like of mambo, most ppl do this, and you get to see some quality djs for free at them.