Depends when you are planning to go.
Ibiza pretty much shuts down end October through to Mid April.
If you are going May / June, then get yourself as far as the UK or Eire, and pop into a travel agent and get a last - minute deal. Have a look on the web before you go.
Package deals were very cheap last summer. (Good year for consumers, crap for my company!) Deals for 2 weeks with accom were going for between 100 and 150 per person, including accom. and a flight from London.
Don't bother with a package from the US. can't see how it could be cheaper.
So much competition in the UK and Eire for packages to Ibiza, in fact to all of the Balearics. For example, if I remember right, Britannia were doing approx 135 flights per week to Majorca. That's a lot of seats to sell and there will be flights most days of the week, so I would not worry about leaving late. As long as you are a bit flexible, you will be fine.
If you want to stay longer, have a look and compare the flight only prices. The holiday companies need to sell the seats on the plane so much, that the accom+flight package prices can be similar to the flight only. If you want to stay for more than 2 weeks, we had loads of people booking a package to "start off" and then finding a cheap apartment to base themselves in whilst they party all summer long.
You will love Ibiza. Great sunsets, great beaches, great nightlife.