VPC Peering is the network connection between two VPCs using private IP to communicate. VPC Peering is useful for setting up a simple network scenario with a limited number of VPCs. In such cases, the direct connections between pairs of VPCs are sufficient to route the traffic. It can be used if the primary purpose is resource sharing between two VPCs.

Auto scaling helps the users to scale the resources automatically with respect to traffic fluctuations by setting up policies that determine when to scale.

Scaling policies are rules that define how the resources should be scaled with respect to some threshold.
Target tracking scaling: Increase and decrease the current capacity of the group based on a Amazon CloudWatch metric and a target value.
Step scaling: Increase and decrease the current capacity of the group based on a set of scaling adjustments, known as step adjustments, that vary based on the size of the alarm breach.
Simple scaling: Increase and decrease the current capacity of the group based on a single scaling adjustment, with a coo ldown period between each scaling activity.

There is no cost associated with setting up auto scaling, but you will be billed according to the resources the are scaled. You need to check the resource's respective pricing to calculated how much will be billed.

AWS Auto Scaling Group helps to improve availability of the application, by dynamic resource allocation and automatically scales with respect to traffic spikes.