This is brilliant stuff, I have it in my rabbit run, and am soon to get the shed lined with it too..
The cost depends on the thicknes of the rubber, ebay is probably a good place to look, other than that, see if you have any local suppliers.
You'll probably need 2 pieces for that area, those I have are 4 x 6.
I'm lucky as I bought mine second hand, I paid £60 for enough for a 12 x 14 stable.
Jx