Look at the "Promos promos promos" thread on here. The x days plus y free tend to work pretty well in New York and I can usually manage a blended average of <$300 plus tax for a nice Manhattan 4* with a bit of work. You may well find the 4+2 good value, although I usually found myself better off with two hotels on a 4 and 3 day stay (or similar). My preferred options are the W Union Square and Times Square.

If you need to go cheaper, the W Downtown, the Sheraton Tribeca and the Westin Grand Central can often be found closer to $200 (though the W in particular can equally be $350+ for no apparent reason).