Bit confused. Couldn't you just run a Linux VM to do your NAT and only pay normal egress?
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_NAT_Ins...
I say sorta because it's built on an old version of Amazon Linux and is headed towards EOL with no replacement except "go build your own" as you suggest.
https://www.lastweekinaws.com/blog/an-alternat-future-we-now...
AlterNAT uses managed NAT Gateways as a fallback when the NAT Instance is out of service, but again you will have to make your own NAT AMI.
This is not to excuse AWS' frankly absurd NATGW pricing, but to point out other ways around it.
https://github.com/somleng/somleng-project/blob/main/infrast...
+ Run extra for failover, HA etc + manage security + Monitor performance + ...