Annoyingly, the VPC NAT Gateway is more expensive than running your own nat instance. If your traffic is low enough to be handled by a t2.small or lower, it's cheaper to run your own. Most of my NATs are nanos or micros.

AWS also double-dips on the traffic charges for the VPC NAT - you're charged for the traffic it transfers, but you're also charged for the same traffic in the general bill, from what I've been able to glean. Given that traffic is where AWS is not competitively priced, it's something to be cautious of.