Best Way to Get 5V, 2A from a 2-Cell LiPo Battery
The cost of the regulator seems about right, and is definitely not too good to be true. But what makes you think that the regulator is all you need? Look at the data sheet you linked to, and look closely at the applications section. You will need several more components, particularly including an inductor which cannot be just any old inductor.  Even worse, you'll need a way to mount all your components, and you cannot just solder them together. Switching supplies are susceptible to layout errors, so a printed circuit board is a really, really good idea.

Between the components and the pcb, for small quantities of finished product (like, let's say 100) you'll almost certainly end up spending more than 16 bucks a unit.

So eBay is probably your best bet.