There is a flat rate in Rim's service documents. I think it was $89. By the time we need new batteries, there will probably be cheaper third party options.
I'm not even worrying about this yet since I've seen no sign of a downward trend in battery health yet. Some time when it's been about a year since Battery Guru was launched (July 20) I'll round up data from people who've sent in export files and see if there's enough to project forward in time to when we'll be getting annoyed with the reduced battery life.
I have a suspicion RIM has designed this (mainly by limiting discharge to 3.4V) such that we'll get well over three years from it, but it's just too soon to say.
I bought an Acer laptop in August 2008 and its battery never even made it to 4 full hours. Since then, it has slowly declined such that now it only lasts about 30 minutes and I constantly get a reminder saying it is time to replace the battery. Haven't found anything for under $100 yet but haven't looked too hard. Since I mostly take my PB with me in my pocket when I go out, the laptop stays at home plugged in.
Based on this recent lithium-ion battery experience with my laptop, I feel confident the PB battery will hang in there till well into 2014, at which point I can get the battery replaced or consign it to some lesser role in the house where it can stay plugged in all the time. By then I'm sure there will be even more amazing BB devices out there for me to choose from.