Hi Zach
I'm not from Adobe, but my understanding is that the ToolBox is a free download. You only have to pay if you want support.
The Workflow SDK is something different. It's an extra bit of functionality for Workflow (including Docs, Samples, and QPACs). You can use it for either the Toolbox or the full product.
As far as I'm aware, it's freely downloadable and useable subject to the terms of your main Workflow license.
Howard
http://www.avoka.com