A single EC2 instance might not cut it. The AWS Managed NAT GW scales up to 45Gbps. They can also support 55k connections to a single destination (multiply that by the number of permutations on your triple - IP addr, destination port, protocol).
If you have single EC2 instance doing the job of a managed NAT, another equivalent EC2 instance is enough to max it out.
You may need a fleet of instances if your requirements are large. Which means that you have a bunch of operational aspects to worry about and the NAT Gateway calculation starts to become more palatable (once you start adding the human cost of maintaining your own, etc).
Pricing is still outrageous though. AWS has economies of scale that we don't.
If you have single EC2 instance doing the job of a managed NAT, another equivalent EC2 instance is enough to max it out.
You may need a fleet of instances if your requirements are large. Which means that you have a bunch of operational aspects to worry about and the NAT Gateway calculation starts to become more palatable (once you start adding the human cost of maintaining your own, etc).
Pricing is still outrageous though. AWS has economies of scale that we don't.