If you're talking the big clubs and the big promoters (eg We Love Sundays, nights @ Pacha etc) then expect to pay around 35 euros a night. If you buy tickets in advance it's cheaper than paying on the door (can be up to 50 euros) and you tend to get free drink if you buy ticket in advance and this can be worth up to 10-15 euros in the likes of Pacha, Space, Amnesia.
Night in the San An clubs like Eden and Es Paradis are cheaper but I couldn't tell you prices as not been.