The Standard has had some pretty good rates, although I think they've been creeping up with all the publicity the Highline has been getting. I'm not sure how it is as a hotel, but as a piece of architecture, I think it's the best thing to hit NYC in ages.

The Cooper Square Hotel has also had some really cheap rates lately, although I don't know if they'll still be good in September. If you stay there, though, don't use the outdoor bar. You've been warned.

If there's something cheap on Expedia and you don't mind prepaying in GBP, PM me with your e-mail address and I can send you a 20GBP referral credit for expedia.co.uk (full disclosure: I get some credit myself if you use it).

You might also want to check out Quikbook, which often has these sorts of rates.