I think I was perhaps just fortunate that the books I was looking to buy at the time seemed to be a bit cheaper at BooksOnBoard, but they are still a viable alternative for me. Kobo generally seems dearer, but not always. Amazon is still my main choice.
For me in the UK, the Twilight book is £4.99 at Amazon, Kobo & BooksOnBoard.
Some random book prices:
The Honey Queen - Cathy Kelly (£8.99 all 3)
Lost and Found - Tom Winter (£6.23 Kobo, £6.86 Amazon, N/A BoB)
The Hidden Cottage - Erica James (£7.99 all 3)
River of Destiny - Barbara Erskine £3.99 Amazon & BoB, £4.99 Kobo)
The Midwife of Hope River - Patricia Harman (£3.30 Amazon, £4.19 Kobo, N/A BoB)
Gone Girl - Gillian Flynn (£4.99 all 3)
Private Berlin - James Patterson (£7.37 BoB, £8.48 Amazon, £8.99 Kobo)
Life of Pi - Yann Martel (20p Amazon, £3.97 Kobo, £6.14 BoB)
The Blackhouse - Peter May (£2.99 Amazon, £5.46 BoB, £6.47 Kobo)
(FWIW, a couple I couldn't find in the Sony Store, and apart from the Life of Pi which Amazon is pricematching with Sony, one was the same price and the rest were all dearer than Amazon). All fairly similarly price at Google Play also.