If you're lucky, sometimes you can find tickets in the 20-25 Euro range at some of the pre-party bars around Ibiza or Playa d'en Bossa, but that often means stopping in for a drink. Cheaper tickets sometimes have restrictions (like you have to arrive before 2am).
Also, the drinks in the clubs cost 6-16 euros (depending on what you drink) so you should factor that in when buying your tickets. In other words, if someone offers you a 30 euro ticket with drink or 25 euros without, get the 30 euro one.