You might want to try Priceline. All the big midtown hotels use them and some real good bargains can be found. Usually you can get 4* hotel for around $100 an ight.
The following list is taken from biddingfortravel.com and list all the known Priceline hotels for midtown:
MIDTOWN WEST
4 Star
Hilton and Towers
Marriott New York Marquis
Renaissance New York
Millennium Broadway
City Club Hotel
Hilton Times Square
Westin Times Square
Sofitel New York
3 Star
Paramount Hotel
Doubletree Guest Suites
Novotel
Sheraton Manhattan
Sheraton Hotel and Towers
The Algonquin
The Mansfield
The Belvedere
Warwick Hotel
The Shoreham
2.5 Star
Club Quarters Midtown
2 Star
Howard Johnson Plaza
Wellington Hotel (On border with Upper Midtown - Central Park South)
MIDTOWN EAST
5 Star
The Waldorf Towers
4 Star
W New York
Waldorf Astoria (managed by Hilton)
Intercontinental The Barclay New York
Grand Hyatt New York
Millennium UN New York Plaza (formerly a Regal Hotel)
3 Star
Radisson East Side
Roosevelt Hotel
Crowne Plaza at the United Nations
Marriott East Side
Roger Smith Hotel
Beekman Tower
Metropolitan Hotel
2.5 Star
Courtyard by Marriott Midtown East
2 Star
Plaza Fifty Suites
Cake or death?