Go now to amazon.com and search The Mortal Instruments: City of Bones; City of Ashes; City of Glass. You will find that the box set of this series is 14.99! It's the cheapest you will find it, and I don't know till when exactly this offer will last so move move move!!
Hope this helped, also happy new years!!
CLICK HERE and it will take you directly to the page.
Other good offers are for The Vampire Academy box set with books 1-4 for $24.41, Blue Bloods 3-Book Box Set for $16.49 and The Black Dagger Brotherhood series books 1-6 for $31.00.
PS; I am not affiliated in any way with amazon, I just thought that some of you would benefit from these offers =D