uptown (upper east and west side, Park Avenue, Turtle Bay etc.) is the most expensive. A ton of new buildings have come on the market all over Manhattan but especially in the Chelsea area of Manhattan, with a brand new 2 bedroom with doorman and high end facilities going for say $3,800 - $4,800. South of Houston St., rents have come in quite substantially, I'd say anywhere from 15-30% since 9/11, so say $300-$600 or so less than Midtown per month.
My in-laws have spent the last 8 weeks or so apartment hunting all over Manhattan and still have not found anything satisfactory. Their budget is less than $5,500 per month. The problem is either space (they want at least 1500 sq. ft. or views (they want dramatic) with a doorman and new or almost new fittings. If you are less picky and especially if you are willing to live downtown, there is a glut of apartments right now.