I don't know if this is a pricing mistake. This is REALLY cheap and you can cut it in half to create two twin size mattresses.
75% savings and cheaper than a Full and queen size mattress?
And if you want to make it even thicker, you can just add mattress toppers to the top.
This is a really good price in comparison to similar spec'd mattresses with 3.0 lb memory foam, 8" thick. Remember, this is for a KING size for $200! There are other models with higher density foam, (ie. 4.0lb or 5.0lb) but they cost more than 50% more!
My recommendation is that you try out this 8", and if you want to try out a denser foam, you can switch out the topper with whatever density you want from a place like: http://www.mattressbymail.com/memorytopper.html
Even if you buy a 4lb Memory Foam topper at 2" to top your existing mattress, it's only $106.99 more, which is still cheaper than the next alternative.
If you would like more information the "theory" of memory foam beds, check out: http://www.gmilburn.ca/2008/04/27/make-your-own-memory-foam-bed/ Once you understand the theory a little more, you can customize your bed for a truly DIY feel!
MAKING TWO TWIN BEDS:
The dimensions of the King Size is about 75" x 80".
So when I got it, I cut it in half to create two 40" x 75" twin size mattresses. I bought a twin size zipper cover off Amazon. Any will do depending on your needs. I needed pee-proof because my two kids were going to use them. Mark it down the middle, and then I had a cheap hacksaw with some very fine teeth, and it cut through fairly easily.
The mattresses even come with a King size zipper cover that I can use for whatever thing I see fit in the future. I actually already have a different mattress I had made back in about 2007 that I'll be able to use it on.