It really depends on your needs - I use nano sized SPOT instances for NAT gateways which only cost a penny a month. They in no way compete with the 40gbs capaci

It really depends on your needs - I use nano sized SPOT instances for NAT gateways which only cost a penny a month. They in no way compete with the 40gbs capacity and high availability of the hardware NAT devices but if the majority of your traffic is internal, going to a peered VPC, or over IPV6 and you just need a means to make an occasional API call to one of the AWS endpoints that don’t yet support IPV6 (which is the majority of them), then it’s a perfectly viable solution - better then sharing a hardware NAT IMO because you can take advantage of network traffic within the same availability zone being free.