I have a third gen Prius also, 185,000 miles or something now, about 150k when I picked mine up 5 years ago. It's been pretty reliable for me but obviously I haven't put a lot of miles on it. The main packs are supposed to generally last 150-200k miles and I've noticed as summer has worn on, mine tends to show the battery quickly run to 2 bars every time I get in the car, taking a couple miles to charge back up to the normal level of 6 or however many bars it normally sits at. Previously it would only run the battery down if I was driving a long time on electric or running the A/C while sitting idle in a parking lot, so I suspect I may be getting down to the last year or two before my main pack gives out. I was seeing quotes up in the $2500-3000 range for warranteed packs with installation, which if it helps me get, say, another 5 years out of the car I'd be fine with.
I've only run up $1741 in repairs and maintenance in the last 5 years and 35,000 miles which I feel is reasonable. Puts my maintenance and repair cost around $0.05/mile, or about half or a third of what I'd been planning for. Then again, a $2500 battery replacement would instantly skyrocket the cost per mile, so there is that to think about. I think that long term it's reasonable to assume $0.10 to $0.15/mile of maintenance including when various other things fail like axle seals, shocks, A/C compressor, and so on. Little things that can randomly fail when you're driving a car to 15 years old or more, especially if parked outside in severe climates.