It should come as no surprise to learn that Ubisoft will be releasing plenty of DLC for Assassin's Creed 3, but the latest PlayStation Store update confirms that you'll be able to buy the complete offering in advance with a Season Pass purchase (via Joystiq). While Ubisoft has yet to clarify exactly how it will work, you'll be able to get the Season Pass at launch either in a bundle with the game or separately through your chosen online storefront.
There's no price for the a la carte offering, but the PS Store update does reveal prices for two AC3 bundles. The Assassin's Creed 3 Gold Edition will sell for $89.99, including both the game and the Season Pass. There's also a $119.99 bundle that adds the PlayStation Vita-exclusive Assassin's Creed 3: Liberation to the Gold Edition mix.
Based on this information, it seems reasonable to expect a $30 price tag for the Season Pass on its own. It's also a safe bet that this offer won't be restricted to Sony platforms. In both cases, we won't know for sure until Ubi actually confirms it.