Would be nice with a low end alternative as well. At the moment I'm running the NAT instance on a t2.micro at $0.013/hour (On demand).
The price of this service starts at $0.045
If your setup can't absorb $30/month, you probably don't need isolated private subnets and highly-available NATs.
Well, they've just released t2.nano.
Annoyingly expensive. I wonder if they have a goal of restricting the number of VPCs, as you basically can't run a VPC without NAT to access other AWS services like S3.
But the t2.nano instances will make a DIY solution cheaper.
What makes it expensive for any significant traffic is "data processing" fee.
But for S3 alone you can get away without NAT by using VPC endpoint.