I didn't think it's especially cheap considering it's Akihabara. BenQ doesn't have a big market there and the brand has no premium value either. NEC, Hitachi-LG, Pioneer, Toshiba, Matsushita-Panasonic are the leaders. (BenQ and Lite-On are Taiwan companies and not strong in Japan. Samsung also has a very small market share.)
Maybe DW1640 is overpriced in Europe. Consider the PX-740 based on DW1640 price in the US market. Some people will probably buy PX-740 for under US$100. (Plextor is that kind of company that buys at US$50 per unit and re-brands and sells for US$200.) I'm sure DW1640 price will drop to under what DW1620 price was at its release time.