Take a look at these sites, for starters -
http://www.mobissimo.comhttp://www.kayak.comBoth sites search an immense numbers of airlines and travel sites - it really helps to see all your options laid out in one giant list. I just ran a quick check on Mobissimo and found direct flights on both Delta (JFK-LAX and back again) and Continental (EWR-LAX and back again) for $284 and $318, respectively. You may be able to find flights that leave from EWR on the 30th and fly into JFK on the 5th, but the cheapest options are likely going to require that you transfer between airports on one of the two days. As long as you leave plenty of time for the transfer, you can do it for as little as $25 per person - or take a taxi for just a bit more (EWR to JFK is $60 flat rate plus tolls). You can find more information about transferring between airports
here.
Definitely have a look around - there are tonnes of options, and most of them are fairly reasonable (more reasonable than I'd have expected, given the late date and the availability of non-stop cross-country flights!).
Good luck, and if you have any other questions, please don't hesitate to ask!