VPC has no cost associated. However, if you want to extend your data center or provide a secure IPSec tunnel through a VPN client, you need to add in costs for the VPN connection and data transfer costs over the VPN tunnel. This is the first place to look at to decipher VPN connection cost: http://aws.amazon.com/vpc/pricing/. The cost is $0.05 per VPN Connection-hour + standard AWS data transfer charges for all data transferred via the VPN Connection. For example, if you have 2 VPN connections the cost for those connections would be: .05x24x365.25x2(2 connections) / 12 = ~73.05 + data transfer.
Now for the data transfer out piece: On data transfer out for VPN connection, the cost is .12 per GB for less then 10 TB a month per connection.. So, two connection each at 1 TB a month would be $245.76 a month.