In short
Ibiza is a World Heritage site for two things that have nothing to do with its reputation: the fortified upper town of Dalt Vila, and the Posidonia seagrass meadow in the Ses Salines nature park, one of the largest in the Mediterranean.
The north of the island — Santa Agnès, Sant Joan, the coves at Benirràs — is nothing like the coast of the big clubs, and many visitors never see it.
What it really costs
Ibiza is the most expensive of the Balearics in summer, and the gap with May or October is threefold on accommodation.
The going-out budget is the item that explodes: entry to a major club costs more than a hotel night out of season, drinks not included. That is a choice, not a given.
What nobody tells you
The Balearics apply a per-night tourist tax, added to the advertised price and paid on site.
The Posidonia meadow is protected and anchoring over it is regulated: boats that drop anchor on it are fined. That plant is what makes the water so clear.
The island has a chronic water shortage and seasonal housing is so tight that season workers sleep in their cars. Renting for a month in July is a matter of forward planning, not searching.