How much will it set you back? Well, Windows 8.1 is available as a free upgrade for existing Windows 8 owners. Free! But if you're one of those people who has been holding off installing it, but are now tempted given the update, then you can snag a copy for $120—or $200 if you'd like the Professional Edition.
In return, you'll find a lot of little changes which make a big difference. And a Start button. What are you waiting for? Go download it.