I've just seen an advert in "Arthur Frommer's Budget Travel" mag (Aug 2001 edition). The Hotel Wolcott in midtown Manhatten has a special offer. All Summer inc. weekdays & weekend they have a rate of $99US$ + tax. You have to mention booking code #1119. You can e-mail them at :
sales@wolcott.com Rooms in the ad are spacious and it says they all include Colour TV, in-room movies, internet access, Nintendo, hair dryer, in-room safe & closet. The website (with photos of the rooms) is: http://www.wolcott.com
Hope this helps!
I don't know about NYC but in the UK you can rent student campus accommodation in the Summer hols really cheaply. Maybe you can in Manhatten? You would need to contact the Visitors bureau/convention I guess.