I wouldn't be surprised if Ginobili is worth $10M per season the next two years. Technically, looking at the metrics that measure the number of wins the Spurs get from Ginobili being on the team and then multiplying that number by the value of a win, Ginobili will have been worth the $14M he got this season.
Considering that Duncan set the bar at $10M per season, I think Ginobili will come in under that. If true, whatever he signs for will be a bargain. $7-8M would be a great deal for the Spurs. Even if he wants the same contract Duncan got, I'd give it to him.
Obviously we all hope Ginobili signs for the lowest amount possible in order to open up salary cap room. But let's not underestimate his true value ... he can still be great at times today and should remain a very good player in the next two seasons.