Is the battery dying or dead?

New situation with my 15-year old Civic today. It had difficulty to start this morning but did start. Then in the afternoon when I got off from work, it was even harder to start but started, I thought the battery may be runnin low & not charged enough so I drove on the highway at 65-70 mph for about 10 miles, and stopped at a store for something for about 20 min, then came out etried to start the car, and tried 3 to 4 times but it did not work. I had to find help to jump start the car and run on highway another 10 miles to home. Checked the battery voltage at 14.24v when engine is running, and 12.48v when engine is off.

The battery is close to 8-years old now, and mainly run 2-3 mile one way to and from work daily in the past one year. Starting difficulty initially began less than two years ago in the very cold winter days but only once or twice, and now gets much worse even in the warm weather of 60F's. 

Stopped by an auto part store early this evening checking battery for the car, seems ethe battery is priced over $105, is this normal?

Thanks for advice in advance.