Prevent DDoS and bill increase - Firestore public data

I am having a charity website, which stores how much money are donated to different projects in a public Firestore database (Read access - public, Write access - none). Basically that is fine for security, but what if a hacker maliciously sends millions of reads to the public documents. That will increase my bill a lot. How can I prevent this from happening, while simultaneously keeping the data publicly accessible.