What title was $15? That is incredible. The highest I saw was the around $8, and it was the two-in-one.
Also, search for "kobo coupons" - there is a 20% off sale going if you look up the code, and also a 35% one-off for the first book you buy. In Canada, with the discounts, I am paying a little more than $5 per book, which I think is a fair price.