This all depends on your storage and server provider as they all apply different pricing and rules.
Some providers charge traffic leaving their infrastructure (called egress) and some charge entering traffic (ingress).
In most ingress data is free.
What about backing up data from a DigitalOcean Droplet to a DigitalOcean Spaces?
Referring to https://docs.digitalocean.com/products/spaces/#bandwidth
> "Traffic from Droplets to Spaces does not count against your Spaces transfer allowance (because inbound bandwidth to Spaces is free), but does currently count against your Droplets’ outbound transfer allowance. We’re working on changing this behavior."
So inbound traffic to DO Spaces is free, but outbound from DO Droplets may incur costs.
Note that If you use our remote/serverless backups option for example to back up your databases, you will not incur any costs (below a certain threshold) since we include some free bandwidth in all of our plans.