Recycle them. As I recall, the recycle rate of lead batteries is around 90+% (or higher---its been a while since I looked up the statistics)---[late note--one source on Google gives the rate as 97.1% between 1997 and 2001]. I suspect the way it will work is you will drive back to the Toyota dealer (for your Prius), they will pull the old batteries and send them to the recycler, and put new ones in.
Will that be expensive? It wouldn't be a lot of fun to invest another couple of grand in 5+ year old car.