It was in an earlier Discussion:
"If you don’t own iLife '11 or iWork ’09 apps (or v1 iOS apps), or own older Mac versions: On the iLife side, you’ll have to buy iMovie and iPhoto at full price ($15 or $5, depending on platform). GarageBand is free for everyone to download, though the app does offer in-app purchases. Apple’s iWork apps are each $20 for the Mac, $10 on the iPhone or iPad." -- Terence Devlin Nov 12, 2013, in
Apple Support Communities > iLife > iPhoto for Mac > Discussions >iPhoto Mavericks Not Free??
But in re-reading the Macworld article he referenced, I believe it is probably only iLife owners who can upgrade iPhoto free.