I agree with Siklong69, in terms of the DB and what we know for sure. I have a feeling though that Yuen Long may be a hot spot for locals with a bit of knowledge.
On the DB there are a fair amount of girls, which to me implies there are lots more since you hardly hear of any punters from this forum going down there.
One issue I have with a lot of the girls in NT is that they often don't show their faces on the DB (Tai Po is much better than other areas in this respect). I guess this means that they live there for a prolonged period (rather than just 1 week like some Hotel Res. girls) and don't want to get made.
I'd say that generally, if you pick carefully, you can get a nice young WG for under $400 in NT. But don't expect the level of service to be great and be prepared to lead the encounter.
I've punted a few times in the NT so you can look at my reports for more specific details. Good luck bro