@Thomas Murnane Thank you for reaching out to Microsoft Q&A. I understand that you are having questions regarding the location for setting up a Virtual Network with VPN Gateway for your use case.
I see that you have Offices in CA and MA and your current Vnet is in South Central US. I see that you want to know if it is more cost effective to have the VNet in East US when compared to South Central US.
As seen from the Pricing Calculator, US East may could be a cheaper region when compared to other regions so it can be an option for you. However, locating resources in a cheaper region should not negate the cost of network ingress and egress or by degraded application performance because of increased latency.
I would suggest you to also verify other details such as Network Latency between your regions and Azure VMs in the locations that you are considering for your services to determine which location suits your need. You can use a tool like PsPing to check for latency. PsPing is a command-line utility for measuring network performance, it implements Ping functionality, TCP ping, latency and bandwidth measurement. Hope this helps.
Please let us know if you have any further questions and we will be glad to assist you further. Thank you!
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