salesforce had their hosting bill jump orders of magnitude after ditching their colocation, it did not save anything and colocation staff were replaced with AWS engineers

nat is free to provide because the infrastructure to have NAT is already there and there is never anything maxing out a switch cluster(most switches sit at ~1% usage since they're overspeced $1,000,000 switches), so other than host CPU time managing interrupts (which is unlikely since all network cards offload this).

sure you could argue that regional NAT might should be priced, but these companies have so much fiber between their datacenters that all of nat usage is probably a rounding error.