Seems reasonable:

If you do decide to opt in to secure backups, you’ll be able to securely back up all of your text messages and the last 45 days’ worth of media for free. If you want to back up your media history beyond 45 days, as well as your message history, we also offer a paid subscription plan for US$1.99 per month.

https://signal.org/blog/introducing-secure-backups/