Have a good look at the site, then get back to us.
ah, what the hell.
In the big clubs, drinks cost anywhere from 6 euros (bottled water at El Divino) to 15 euros (red bull & vodka at Space).
In the pre-party bars in the major commercial areas (San An, Playa d'en Bossa, Ibiza Town), you'll find the drinks cheaper. From around 3 euros for a pint to 9 for a red bull & vodka.
There's cheaper bars in places like San Antonio's West End and the sidestreets in Playa d'en Bossa. One place I know in PdB will give you a liter of red bull & vodka(?) for 5 euros.