Where should I buy Windows?

So I have built my PC and everything works perfectly, now all I have to do is put windows on there. Does anyone know where I can find it for a reasonable price below $100 I remember finding 7 for $30 back in 2011 but I can't seem to find 8 anywhere near that. I know it's out there for a steal, and I'm not cheap. I'm just broke after building this PC :P 

G2A offers pretty cheap legal codes. Also if your a student, or know a current college or highschool student, copies can often be found for $20 if not free with discounts through your school. 


There's also reddit.


You could try that thing that microsoft is doing where you can upgrade from 7/8 to 10 for free. Get a code and download it. Put it on a flash drive, and use it to install it on your new pc. Just spitballin', though. Probably won't work.


Use the Windows 10 Tech preview unless you need SLI.