Wonder no more! The rate is based on the competition and the time of year as well as a rate analysis done for the hotels (not just Marriott) by third parties.
As a comparison I plunked down 17 bucks for an average cheeseburger in the Soho area of Manhattan last month! It is not a cheap place to do anything that is for sure.
Now to contradict myself (which I often do) there are serious discounts to be had at some of the Botique hotels like the Belvedere and the Essalyn--check them out. The latter offers free happy hour and free continental breakfast according to their web sites.
And Marriott often offers discounts that drop the price below 400 even at the Marriott Marquis. Shop around--it's not cheap but it can be cheaper!