The Nexus S battery is a pretty standard battery (no NFC antenna or anything fancy). I've bought packs of 3 batterys for like 10 dollars on ebay. They work fine.
The exact ebay page I bought from is no longer there, so I wont provide a direct link. But search on ebay for
and you will find several that sell a wall charger plus 2-3 batteries for less than 15 dollars.
I have compared these batteries with my original...and seriously they are identical (same text and all)...dont know if OEM or fake...but they work.