I can't give you definite answers, but...
> How long does the battery last on a TUNGSTEN W?
It depends what you do. If you have the backlight enabled, more power will
be used. If you do heavy and persistent computation, then more power will
be used as well. More factors leading to battery drain: sound, bluetooth
enabled (anybody feel free to correct me) and even weather (environmental)
> Can you exchange batteries easily?
They are difficult to change unless you feel confident opening up your
device. I don't know where to get batteries, but they are designed to last
enough for a regular 'lifetime of use' (like the battery of the clock on
your PC, for example). If you use it heavily, get one of those battery
> How much do they cost?
Probably not as much as the expertice of people who will change it for you.
It's not cheap in any case.