This is pretty much where my money went, i was out there for 8 nights, and wasn't really watching the pennies too hard, tho i probably should have because none of my cards are working at the moment and i owe my mate some cash...
1 x Nice, square meal per day with drink = 10 - 15 Euros
1 x Entry to decent club night = 20 - 45 Euros, sometimes get free drink or bus - i.e Armada
1 x Taxi to San Rafael = 20ish Euros per car
1 x Taxi to Playa D'en Bossa = 30ish Euros per car
1 x (LARGE) Vodka Mixer: Privilege = 10 Euros
Amnesia = 13 Euros
Space = 12 Euros
Pacha = 12 Euros
Bar M / Mambo etc. around 7 - 8 Euros
1 x Shit tiny bottle of water in a club = 8 Euros - by far the worst thing about Ibiza.
1 x Bus home = 1.5ish Euros
I think the key is if you want to drink and not spend a load of cash, obviously get shitfaced before you go to the clubs either in cheap bars or at your accomodation. You can always wangle free shots and drinks out of bars you visit regularly.
If you're careful i reckon you could enjoy a full week on around 500 - 600 Euros, without being too flash or too scabby with your cash.
Ps -Dont buy a vodka redbull in Space - it's 17 Euros (£13 - i nearly cried)