In terms of clubbing in summer = expensive, the "superclubs" often charge around 30-50 euros entry, and perhaps 10 for a drink. Accomodation isn't too bad if you book ahead. Food and alcohol in supermarkets was pretty reasonable considering the place.
For spring it's probably a lot cheaper, but I'd also say it's much quieter and it wouldn't really be the same experience that Ibiza is renowned for. In my opinion unless it's July/August (and maybe the shoulders), you're probably better off sticking to mainland or coastal Spain as it's a lot easier to access and there's probably a similar vibe, for clubbing anyway. Or try other popular lively European cities eg. Berlin, Paris, London etc. all of which I'd guess would have better nightlife in the springtime.
That being said, if you decide to go in the summer - I just went last summer and if you're interested in the scene it's worth every penny! There was a great atmosphere there!
And one last thing - it's not really the backpacker type scene (probably due to its price and poser-like attractions), so it'd be best to go with friends or others who are interested in the same thing. Keep that in mind!
[ Edit: Edited on Nov 5, 2008, at 2:16 AM by Erik85 ]