It's a one-off price. Storage Space Direct is a bundled feature of the Windows server. There are no on-going costs and of course, you can purchase the software through it and enjoy the free upgrades. There are no underlying hidden costs, etc. because it is all included in Windows.

If you still use VMware as your hypervisor, you should consider Hyper-V. Since 2012 R2, it is as good as VMware, and with S2D it is much more cost effective.

Cost-wise the product is one of the more affordable within the category of products.  

With the data center licensing and everything that is connected to that, this solution is relatively costly.