1. salary cap goes -> salaries go up
2. rookie contracts do not go up
3. rookie contracts become comparatively better value for money.
it's not rocket science.
Having said this, I'm not sure "their value will go through the roof" as Cuban says.
First, the value of picks depends on many other factors too, and Cuban's claim is a very broad generalization. A top 10 pick is one thing, a second rounder quite another.
Second, they were very cheap to begin with anyway, ie even now in terms of the % of any given team's payroll they are insignificantly low.