No sane person is going to carry around an external brick to save money. Additionally, most of us who want a bigger capacity battery don't LIKE carrying additional accessories. I mean, lets be honest. If the 5000mah battery was really an option, I would probaby carry my PSP and a thinner android device around instead. And seriously, 100$ for a 2.5x capacity battery is par for the course. Im not saying that I think thats a fair price, Im just saying, people are prepared to pay setting market value. Not every decision for every person hinges around cost.
"...You might as well paint yourself yellow, run around like a maniac and call yourself banana man, banana man, cause thats what you're doing..."