Upgrade to Windows 8.1 Pro On The Cheap

Hello All,
I recently built a little desktop computer and I installed Windows 8 64 bit from an OEM disc. I have since updated the system to Windows 8.1 but now I would like to upgrade to Windows 8.1 Pro. I know I need a new registration key, but what would be cheapest way to but the upgrade key and would an upgrade work since I started with an OEM disc? I really am confused about where to go from here.

To do that you would need the Pro Pack:
Buy Windows 8.1 Pro Pack - Microsoft Store

Also see this:
Add features - Microsoft Windows

Thanks. I guess that there is no cheaper way to just upgrade? I will have to look at these options.

currently $92 at amazon Amazon.com: Microsoft Windows 8.1 Pro Pack (Win 8.1 to Win 8.1 Pro Upgrade) - Key Card

If someone knows of significantly cheaper option, please post it. I also will be looking to upgrade to pro in near future.

There are plenty of free options for this, but those are not welcomed on this forum.