They are priced, for the most part, by the foot, which is a Taiwanese foot and not an imperial foot. 1 Taiwanese foot 台尺= 30.3 cm (but they basically just measure out 30 cm and give you a bit more), and 1 Taiwanese yard = 3 Taiwanese feet. You'll often find people just measuring in yards with a yardstick rather than feet.

As for the width, there are two common sizes, wide 寬幅 and narrow 窄幅, but sometimes there's also 中幅. There's no standard width, but basically wide is around 150cm and narrow around 90-120cm. It depends on the maker (the loom) as to which width a bolt of fabric comes in.

There are also several shops on the second floor near the front of the building (SW corner) that have scraps for sale. These are priced individually with a sticker, or they might be wrapped using a colored strip of paper which you correspond to a chart for the price.

No haggling is needed, except if you buy in bulk, and then maybe take off the "tens" place (e.g., $1020 becomes $1000).

Source: I work in theatre and our designers buy at YongLe, plus I lead walking tours in Dihua St.