I went to Ibiza at the end of September last year and had a great time. It's not as busy as August, but that's probably a good thing considering people complain about how busy August usually is.

If you're considering not going because it will be dead, don't worry. It won't be.

I think you're going at a great time close to all the closing parties. 3 days is CLEARLY not enough time to experience Ibiza, but it's better than nothing! It'll give you a good taste of what's to come in your future ibiza travels... and i guarantee you will have them! lol
I don't really think 500$ Eur will be enough to be honest. You gotta remember that entrance to these clubs range to 30$-60$Eur, drinks/ water are $8-10 Eur/ bottle, and cabs are 20$ Eur per way. Accommodations can be cheap but then you're too far from everything and your cab rides would cost too much. If you're planning on eating, it can get really expensive. But depending on how cheap you can find your flight and accommodations, it can be done.
Bottom line is that it's worth it, no matter how much you spend cuz you're going at a great time

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"You don't know if it's fear or desire, danger the drug that takes you higher..."