I live in Shenzhen now, and I've had tons of clothes made here, with varying success. As I'm sure you're aware, the tailors are all concentrated in the Luohu train station. Tailoring quality varies wildly from tailor to tailor and sometimes from garment to garment made by the same tailor. If you can afford it, get a suit made in Hong Kong, it will without a doubt be worth it. That said, I've helped one of my friends get a pretty decent suit three-piece hand-sewn, fully canvassed (from what I could tell) suit made for under $100 including fabric. Speaking of fabric, that's one of the limiting factors. It's just hard to find quality fabric among all the crap.
If you live in Shenzhen, I'd be happy to show you the ins and outs. The fabric market can be a little overwhelming. This of course is only if you can meet me sometime before April 9--I'm moving to India that day.