What is the cost of a beer in ibiza?

That will depend on the brand of beer and where you are buying it, neither of which you have specified. Even in the same town, you will pay different prices for the same beer. Even in the same bar or hotel, the price may change during the evening. You can also have different variations of the same beer, like being draught, bottled or canned. So there are a lot of things to consider so there is no single answer to your question as it stands.