Very cheap suits can be had both "off the rack" and from tailors. Off the rack, made in China, suits can be had for a few thousand, although the materials and cut may not be to your liking. Alternatively, the tailor down the street said that she could make me a suit for less than 4,000 (44R or 46R). She has quite an array of fabrics and a fairly good reputation. I am sure that if you are willing to shell out a few more clams for your bespoke suit, you would end up with a very nice garment. [warning: these prices are based upon my experiences in Tainan]
If you time your purchase correctly, or have a good relationship with department store sales staff, you can also find higher-end suits at reasonable prices. I have found good deals at Shin Kong Mitsukoshi on last-season fashions (i.e., a quality Emanuel Ungaro suit for under 10,000) or special holiday offers (i.e., a gorgeous Daks three-piece for around 20,000). Alterations on such suits may run you 1,000 to 2,000, but are worthwhile for a good fit.