Note dragons have all good saves, so it should just be 7+stat+other bonuses (say from a Great Fortitude/Iron Will/Lightning Reflexes feats if you take them).
Also note @omni-mage, the prestige class difference is meant to account for half the benefit (or relative detriment) of good and bad saves because you might be going from a bad save to a good save in the prestige class or vice versa, and this was standardized before Fractional Base Bonuses was made a specific optional rule. In any case where someone is multiclassing, even into a PRC, I recommend just applying the FBB rules for simplicity sake.
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d12 Hit Die.
Base attack bonus equal to total Hit Dice (fast progression).
Good Fortitude, Reflex, and Will Saves.
Skill points equal to 6 + Int modifier (minimum 1) per Hit Die. The following are class skills for dragons: Appraise, Bluff, Climb, Craft, Diplomacy, Fly, Heal, Intimidate, Knowledge (all), Linguistics, Perception, Sense Motive, Spellcraft, Stealth, Survival, Swim, and Use Magic Device.
Traits
A dragon possesses the following traits (unless otherwise noted in a creature’s entry).
Darkvision 60 feet and low-light vision.
Immunity to magic sleep effects and paralysis effects.
Proficient with its natural weapons only unless humanoid in form (or capable of assuming humanoid form), in which case proficient with all simple weapons and any weapons mentioned in its entry.
Proficient with no armor.
Dragons breathe, eat, and sleep.
You could also look at any class entry and figure out the progression (same as what Omnimage stated). Classes either have good or bad save progression for each kind of save. Fighter for example has good fort and bad will/reflex saves. At level 20 the fort save is 12 (20/2+2). At level 10 the fort save is 7 (10/2+2). The reflex and wisdom saves are 6 at level 20 (20/3) and 3 at 10th level (10/3). Do note that they are rounding down saves to the nearest whole number.
If you just imagine your dragon HD as class levels, that will tell you where you should look on the chart for saves.