First tried this sauce on a trip to the US and brought a bottle back which i went through very quickly. I looked on Amazon and its comes to around £7 (Inc. delivery) which is bit more than i wanted to pay.
I was in Iceland on Narborough Road in Leicester today and got it for £2.50. I'm not sure if it is the absolute cheapest but i've not noticed it available anywhere else. Not sure if its store specific.