startup battery

I have a 2008 Prius that the battery has never been replaced. The battery died the other day that I had to jump-start it to drive. I noticed that the battery to drive the car was actually close to full so it seems the battery to start the car is totally unrelatd to the battery for driving the car.
I suspect that the capacity for store power for the battery is low. Is it self-replaceable? How much does it cost to replace it? I read that the other battery costs over $3000 to replace.
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