This phone is around 30 euros before taxes, unlocked and free. I don't think the battery is particularly larger than in other models, it's just that the phone itself is quite economic in its use of it, I guess. There are few features and it uses simpler software.

Batteries are getting better all the time so this should be a case of the new, modern battery and simple technology which gives it prolonged use. If they really wanted I would bet they could make it to hold even longer before charges. It is a matter of tradeoffs as you say.

I have a bigger issue with the fact many people consider such phones are for poor people or for developing countries. I for the fact, want such a phone on purpose. Hell I wouldn't even mind the colorless screen. This is what irks me. Such phones are becoming increasingly rare and difficult to buy. I do not want a camera, mp3 player, FM radio, browser, apps, MMS and gazillion other gimmicks. I want to make calls and send/receive SMS without charging it every few damn days.