Original DELL batteries sell for a lot less in some countries/places, like
www.dell.com.hk (sorry can't direct link - select "Handheld" from the dropdowns, then click "Handhelds Accessories" on the left about half-way down the page ) HK$69 = US$8.90 including local delivery, directly from DELL. Notice that the 2200mAh one is 4 times as expensive.
So theoretically it is possible for people to sell the stuff a lot cheaper on eBay and what-not. Question is how do you know these 3rd party sellers do in fact sell the real stuff.