So, the Liberty will be able to put about 20,000 kg into LEO for about $9,000 per Kg. The Falcon 9 can put just over half that (10K kg or so) into LEO for somewhere between $5,400 - $6,000 per kg, depending on the load factor. (Numbers pulled from the SpaceX web site.)
Of course, there are other costs besides the raw launch cost (insurance, etc.), but it will be interesting to see how these two vehicles compete. For things like ISS resupply missions, it may make sense to just shoot the Falcon twice.
Once the Falcon 9 heavy gets into the mix (32,000 kg to LEO for $95M), ATK & Astrium will need to sharpen their pencils a bit. That'll be one and a half times the payload for half the cost or so.
Price wars for space launch capacity? I can't wait to watch!