Kobo had the 90% off sale last April in addition to the $5 and $10 off promos. I also believe those were used to inflate sales for prospective buyers/Rakuten.
Also, Kobo has to pay Adobe fee (22 cents) and transaction costs for each title. My credit card costs retailers 3% plus a swipe fee and since Kobo doesn't have a cart all those individual swipe fees add up. And Paypal has additional costs.