(Azure DevOps PM here) Test Manager comes with VS Enterprise, VS Test Professional, MSDN Subscriptions and is also available for direct purchase through https://marketplace.visualstudio.com/items?itemName=ms.vss-t.... You are welcome to take your pick from what suits your needs.
As a subscription, it's more per month than Visual Studio Professional. As part of VS Test Professional it's almost twice as much for the first year. And you can't really do any test plans without it so its a huge part of DevOps that is just unused for us, unfortunately.