Hi Steve,
Based on my experience, the cost of the restoration test is more on the VMs' compute resource and disk recovered in the second region. You need to count them with hours or days you need for App team to test, that's the biggest part as double the spending on VMs. The recovery vault itself didn't cost too much, you can find the price here: https://azure.microsoft.com/en-us/pricing/details/backup/. Storage could be the major part of your backup. Plus, take note if you restored VMs in an isolated Vnet (if want to keep the same ip), then you need to setup Azure Bastion to let people connect to VMs via the Azure portal, which could cost several hundred dollars per month! https://azure.microsoft.com/en-us/pricing/details/azure-bastion/