Surprisingly for many, Microsoft released the upgrade to Windows 10 completly free. But this only applies, if you already own a copy of Windows 8 or 7. If you are installing a new PC without an OS or want to upgrade a non-licenced version of Windows (because you pirated Windows 8 or 7), you have to pay! Like.. A LOT.
Windows 10 Home costs 119$ on Amazon at the moment, the Pro Version is 199$. With this money you could buy 154 Cheeseburgers at McDonald's by the way.

Here's how you get Windows 10 Pro for just 30 Bucks leaving you 169$ to buy food with:

  • Make sure, Microsoft is still giving out free upgrades to Windows 10 at the time you are reading this:
    http://www.microsoft.com/en-us/windows/windows-10-upgrade

  • Buy a cheap Windows 8 Licence at an online Keyshop. I usually use allkeyshop.com to find the cheapest one. And I'm not even getting Money for linking them...

    Despite the sometimes shady looks of keyshop sites like this, you are buying a legal version of Windows which works without using a crack.

  • If you are installing a new PC, create an Installation Medium as described in Microsoft's Guide

-OR-

  • If you currently have an unlicensed Windows 8, activate it:

    • Go to "Settings" in the Windows 8 Sidebar (Or "Charm Bar" as Microsoft calls it. Great Name)
    • Click on activate Windows and put in your new Product Key.

      Now you should have a licenced Windows 8.1 on your PC. Continue here:
  • Download the Windows 10 Setup Tool:

    With this tool, you can either upgrade your Windows 8 to an equal version (Pro, Home and so on) of Windows 10

  • Let it download the OS, go through the installation progress and get a legal, cheap Windows 10.

I hope you enjoy your extra Cheeseburgers and your new OS!