Firstly I'd try any Windows keys you already happen to have lying around, old keys down to Windows 7 will work to activate current Windows. Otherwise, yeah, maybe look into educational discounts next, technically this can even be the preferable option as I believe the version provided is more akin to the Pro version of Windows. Otherwise as mentioned, unlike with Win7, you can use an unactivated copy indefinitely, with the tradeoff of having a watermark and not being able to change the default wallpaper. In the end, Microsoft would prefer people to use their software to maintain market position, and as such even a non-paying customer is better to them than one who runs Linux or MacOS.
Grey market keys are "risky" in that depending on the legality of the source, they may be revoked, maybe a month after activating, maybe years after. And the resellers typically won't be open about how their keys were sourced, so it's typically impractical to be picky. However plenty of people reason that even if the key is revoked every now and then, at $10-20 a pop it may end up cheaper even in the long run.