As you can read in this article, last year I bought a new regular 2100 mAh battery for my Nexus 4 - LG E960 smartphone, it costed me only a few bucks (9.73 USD exactly).
This week I bought another one, just for my curiosity, from here, but this time a 3500 mAh high density battery, and I paid US $10.49 for it!
I just installed it inside my phone and run then CPU-Z to find the real capacity. And guess what? ;) :) Take a look at this printscreen: