There are various types of king size mattresses and the price varies accordingly. A king size pillow top mattress can be purchased for anything from about $480 to nearly $600. A king size pocket mattress is about $860. At the higher end there is always the Osteo-Therapy mattress which provides extra support for the spine. This type of mattress retails for about $1600 but a good mattress can be purchased second hand for as little as $300.