Any reason why those dates? And any particular airport?
I am assuming you mean New York City not New York State somewhere. JFK is the best airport to fly into if you get that option. It's a set price of $45 (or was last time I was there about 3 years ago - I am going in February so will let you know if it has changed) for a taxi from the airport and you can go anywhere in Manhatten.
Newark is ok if you want to do public transport.
Never been into LaGuardia but those are usually the most expensive flights anyway.
The prices have been going up and up for New York for the past 4 years. When I lived there I could pick up a flight for <£350 and my February flights cost much more than that.
As for dates, New York is unbelievably hot in the Summer, April-May, September-October is the best time. And of course any date you go that isn't a school holiday will also reduce the price....