Hard to give specific advice since you haven't given an actual date. October is a school holiday time in much of Europe so holiday destinations like Ibiza can be expensive depending on date.

Take a look at the Wiki for Ibiza connections and pick a budget flight to a hub such as LHR, CDG, AMS, MAD, then see if you can get a better deal to California from there.

Direct from Europe to LAX will normally be cheapest in most cases, maybe not all.