A recent blog notes that the 8.1 update will be free and available through the Windows Store.
Look out around the 26th June for a public preview:
Ref:
Link Removed
applications
consumer experience
continual updates
dell xps 12
development conference
device choice
free update
game apps
lenovo yoga
mobile computing
new features
oem partners
performance update
public preview
sony vaio tap
tabletexperience
touch experience
windows 8
windows 8.1
windows store