Now that OS X 10.9.x Mavericks upgrade has already been made available for free, it might not be appropriate to answer or comment on this question. Apple already announced on 22nd October, 2013, that this upgrade would be free to all Mac's that support OS X Mavericks.
It was made available free this time, because Apple has more gains with this upgrade over the customers and so Apple wanted everyone (as far as platform support) use Maverics, and encourage buying books. It is worth noting that all the owners of Macs have already paid for OS X, of course for an earlier version. And
The biggest feature that is going with Maverics is availability of iBooks. Apple, for the first time, recommending reading books on laptop, thanks to the retina displays, so you will be able to read the books you puchased in your other Apple products. It is now very clear who gains more with this release, Apple or the consumer - probably a win-win!
In addition to iBooks, the other very important feature is App-Nap, I would rather call it, performance fixes, but I should really appreciate Apple for its generosity for giving away all the performance upgrades (and hence better battery life - when you consume less CPU - you consume less power) to all possible consumers, without even consumers asking for it. No other company would otherwise do this. So with Maverics, both Apple and the end-users have the same gains.
Coming to the other updates to applications, such as iTunes -- these are mandatory updates to leverage the Power-Nap feature and integration with iBooks. However, updates to Safari is anyway available to everyone, including Microsoft Windows users, needless to say why Safari is available to Microsoft Platform and the competition between browser platforms, especially among, Chrome, Safari and IE. What I mean to say, is all these updates are either include mandatory integration with iBooks or with App Nap.
In summary, OS X Maverics is a greate upgrade, unless Apple wanted to push everyone iBooks and performance fixes, a consumer will be ready to pay 50USD to 100 USD (ie., INR 2,500 to INR 5,000) to this update, if Apple had not announced it free.