Buy Windows 10 Pro OEM Licenses for under $30 / license.
Windows OEM licenses can be used on new PCs built by a system builder, which also can be you. If you buy PC components and you build a PC then you can use a Windows OEM license on that PC. Please note that the OEM license cannot be moved to another PC, it sticks to the first installed motherboard. For more information please read the OEM license agreement at Microsoft website.
Here you can buy legit Windows 10 Professional 32-bit and 64-bit OEM licenses at a discounted price of under $30
Kinguin.net is a major discounted Game sales site, but they also sell Windows OEM licenses.
From the Kinguin.net sellers list please choose the one with the best reviews and/or most sales.