To be more accurate regarding the above, a 9th level Wizard should easily be able to get an Int of 28. This translates to a save DC of 24, for a 5th level spell.
If a Balor's Will save is +19, he'll need to roll 5 or above to save, ie he'll fail his save 25% of the time. Cast the spell 5 times in a row, and he'll fail it around 76% of the time.
Of course you won't be binding Balors at 9th level, since you'll only be able to bind creatures of up to 6HD. From the SRD, the Demon with 6HD or under that has the highest will save is the Succubus, which has a Will save of +7. Ie, our lovely Succubus subject needs to roll 17 or above to save. Chances are she'll fail.
In fact, given that Planar Binding only goes up to creatures of 18HD, you'll never need to worry about something like a Balor anyways, unless you're using Gate to bring them in. A 17th level Wizard, with unlimited time to prepare, knowing exactly what they're about to do, should be unstoppable, to be blunt.