Update #1 It seems like not all can see the actual discounted price mentioned below. Something's not right. The discount could depend on your location. I'm in Singapore and I'm looking at US $10.54. Readers from USA say they are looking at US $18. Bummer. Sorry about that.
Update #2: Reply from Book Depository:
We strive to offer our customers the best, locally relevant prices.
Selling conditions, regulations, and restrictions on the books that can be sold and the prices at which they can be sold differ between markets. Similarly, the cost at which we can promptly source and ship product can vary. All these factors are reflected in the prices displayed to customers on our website.
We will always do our absolute best to keep the prices we offer as low as we possibly can as far as is possible in any given market.
Usual price on Amazon is USD $16 - 18.
Usual price on Book Depository is USD $13+.