Cheapest 64-bit Windows

Solution
The cheapest version would be Windows 7 Home Basic edition OEM if you can find a shop that will let you have it and the only reason why it is cheaper than the others is because newer Windows version no longer have the Home Basic variant.

If you want a retail version, then it does not matter where you buy it from since every version of the same Windows tier carries the same list price from Microsoft. It would not make sense for them to have different prices when 7/8/8.1 have free upgrades to 10 since that would make old Windows versions discounted copies of Windows 10 by proxy.


Whatever you find from Amazon/Newegg/TigerDirect/Microsoft/Microcenter.
They are all about the same

There is no 'cheap'.
 
The cheapest version would be Windows 7 Home Basic edition OEM if you can find a shop that will let you have it and the only reason why it is cheaper than the others is because newer Windows version no longer have the Home Basic variant.

If you want a retail version, then it does not matter where you buy it from since every version of the same Windows tier carries the same list price from Microsoft. It would not make sense for them to have different prices when 7/8/8.1 have free upgrades to 10 since that would make old Windows versions discounted copies of Windows 10 by proxy.
 
Solution