To come up with our on-prem compute costs, we baked in the cost of power, real estate, staff, taxes, network infrastructure, servers (both in-use and in reserve), etc. On the AWS side, we used 3 year RIs and Savings Plan. After all that, there was around a 30% cost advantage on-prem. That's non-trivial, but not as big as one might think.

Outbound networking, however, is ludicrously cheaper on-prem. It's about 85% cheaper on-prem than in AWS. Bandwidth is not expensive outside the public cloud.

In fact, egress volume is the #1 cost driver for us moving a service on-prem or building it there to begin with. Some of the AWS managed services are also very pricey, but nowhere near the egregious markup of egress bandwidth.