Casio fx-50FH 計數機 邊度可以買到

I don't know the pricing at 元朗區, but I believe it shouldn't cost more than $150. Meanwhile, the best place to get these calculators is in 旺角.
There are two places to check out:
a. Chung Nam book store
b. HK Electronic OA Centre http://www.hkeoac.com.hk/default.asp
Note that Casio calculators sold at (a) are warranted by Multi-M while those sold at (b) are by Onflo.