Ok 1, like others have said on here $120 is not a lot for an OS. That's the cost of two new games and it runs literately everything on your computer. It's the price of not using Linux. If you're willing to put down the cash for the hardware, be willing to put down the cash for the single most important piece of software you'll run on that hardware.
2. If you go with an OEM copy of Windows 10 home you can get it for under $120. OEM has more limitations with its licensing (tied to hardware its installed on) than a Retail copy. This link here will show you some vendors to pick from. Keep in mind your install media type. If you have an optical drive, it doesn't matter, you can go with CD or flash drive. If you don't have an optical drive, either pay more for the USB version or borrow/find a computer with an optical that you can create a bootable USB drive on with the CD install media.
https://pcpartpicker.com/product/wtgPxr/microsoft-os-kw900140
3. If you're a student, you may have access to a free copy of Windows. Check with your school.
4. I believe you can still use an older OS (Win 7 & 8.1) and use that key to install/upgrade to 10. Win7 and 8 tends to run less for a home copy. However I'm not 100% sure on this since the whole free upgrade was supposed to end a while ago but people keep reporting that they can still do this.
5. You will find REALLY cheap prices on copies from some online sellers. Do NOT buy from these vendors. These are typically people selling pulled keys from systems. The legality is at best gray and often ends up with they key getting banned at some point. I really only mention this because someone on these kinds of posts always mentions "Buy it here for super cheap" and it's something you want to avoid.