Windows 8.1 upgrade to windows 10?

Helping a friend build a computer and his budget is tight for what we want. I was wondering, considering that windows 10 came as a free upgrade to windows 8.1 users, and as windows 8.1 is like 50 bucks cheaper than windows 10. Wouldn't it be possible to buy a copy of windows 8.1 and just "upgrade" to windows 10?