Luckily the upgrade is FOR FREE. I'm already using the new official Windows 8.1 PRO. The Upgrade did not delete my data- I've kept them. My opinion, Microsoft did a fantastic job! The new Windows 8.1 is amazing- compared to Windows 8. Gush, I've finally started using Metro Apps!
The price: Depends on where you are- REALLY! It's the same with Windows 8- Like Freddy already said, you can expect to pay 100 bucks for the Standard version and 200 for the PRO version.
When Windows 8 came out I remember Microsoft made some promotions, so maybe, just maybe they'll do this again. Capitalism, you know- they want people to get Windows 8.1. ^^
Postivies: I like the new Skydrive integration and the renewed system interface(search- yea, they finally improved the search, interface, customization etc)