What's your frame of reference for having "good battery life"? Your own opinion? A Nokia 3310?
Having both the hardware and software closely developed together by one company (Google) will make an impact on battery life. Stock Android having less feature-creep will also make an impact. And finally, Android 4.4 may be slightly more power efficient than 4.3, also making an impact.
I still don't expect the Nexus 5 to outperform monster batteries like the 3000 mAh unit in the G2, but it will have good enough battery life for the masses (and for being a $349 mass market phone). The unsubsidized price of the G2, S4 and other phones with larger batteries is $300 more than the unsubsidized price of the Nexus 5.
I guess you could say it's my own opinion. A smartphone is meant to be a MOBILE device. It's mean to be USED not just sit in my pocket. If you had to always be close to a charger every few hours what the point of even having one?
The problem is that you even consider 3,000mAh as a monster. For the screens that are going into phone now that should be considered average or even small. The Nexus 4 was designed by LG and Google together and it didn't get decent battery life at all. The reviews and personal experience I've read about the Nexus 4 have shown mostly negative comments on battery life.
Also, the cost of the device really has nothing to do with what size battery they put in the phone. If LG/Google would have used the 3,000mAh battery in the Nexus 5 the total production cost would have increased maybe $2-3.
Please tell me what you would consider "good" battery life? How long would you want to be able between charges? How much screen on time would you expect during that off the charger time?