Saving against illusions

[Viconus]

I have a player hell-bent on playing a gnome illusionist.

The rules say you get a save when you interact with an illusion, and that the DM can modify saves based on how realistic the concept is. However, it doesn't establish what the saving throw is.

Is it 10 + the level of the spell + your bonus?

http://www.d20srd.org/srd/magicOverview/spellDescriptions.htm

[Reshar]

Yes, that's the spell's base DC.