The most cost-effective solution to transfer 600 TB of data within 2 weeks with encryption in transit is to use the AWS Snow Family devices, specifically the AWS Snowball Edge Storage Optimized devices. AWS Snowball Edge devices are designed for large-scale data transfers and ensure data security during transit. Given the constraints of a 100 Mbps internet connection, transferring such a large volume of data over the internet would take too long. The Snowball Edge devices can be used to physically transfer data much faster, meeting the 2-week deadline efficiently.