Specifically, egress from Cloud Providers (transmitting data from inside the cloud to a computer outside the cloud) is the usual culprit for being "very expensive". It's part of a pricing strategy by cloud providers that encourages folks to put all their infrastructure inside of the cloud and to have none of it outside the cloud. As one example of the ink spilled over this topic, see this discussion from 2 years ago about AWS's very high egress fees:

Article "AWS’s Egregious Egress" (2021) https://blog.cloudflare.com/aws-egregious-egress/

HN discussion: https://news.ycombinator.com/item?id=27930151

Ingress can also be costly especially if there is a steady state of high load traffic sent from on-prem machines to machines inside cloud (not sure about aws but have experienced this with azure where we had to resort to buy their expressroute which was very costly and ultimately unsustainable for us)

Ingress is free for AWS. Doesn’t make any sense to charge for ingress…

Something else I was caught out on recently is cross availability zone traffic, especially for multi AZ kubernetes where the nodes are very chatty. It's possible to get the services to consider network topology to limit cross az traffic, but it's not the default.