I think there is a perception amongst users that Apple is going to follow the trend of previous years and price Mavericks cheaper than Mountain Lion. In effect, the expectation is that Mavericks is going to be $10 (as 10.8 was $10 cheaper than 10.7). However, by bundling iWork as part of 10.9, Apple can justify keeping prices the same this year if they wish.
That’s the Mac story. On iOS, free iWork is a much bigger deal. Even though it is completely irrational, I have seen a lot of people complain that, after purchasing a $500 iPad, they have to purchase a $10 word processor. ‘Including’ Pages with the iPad is a big draw for consumers — it removes a psychological barrier.