I have a Lumia and I love it, but Paul, that's quite an ignorant response. The ATIV S is a pretty good device. All Windows Phones have their Pros and Cons, don't hate on a device just because of brand.
Anyways, I think that usually the phones sold at cheapest prices are at Ebay for used. Be careful when buying from them as sellers may sell you a non-working phone. Always check.
If you're buying on-contract, there would be plenty of sites selling it for cheap. I'm going to guess you're not though, and in that case a Bing/Google Shopping search may provide you an answer!