I'm surprised the have lasted that long. You know when then are dead when your Date/Time no longer are correct when you turn on the machine. I do not know if they will leak or not from age though. So it may not be a bad idea to replace.
From memory the battery is a 1/2 AA 3V. Looks like a AA but half the length. They cost under $10 a battery but you can get the a bit cheaper in quantity. Last batch I bought was a few years ago. I think I bought 6 to replace all the dead batteries on the Macs I was selling. In quantity I paid around $4.00 each for them at the time.
Macmini G4-1.42, 1GB RAM, MorphOS3.9