At this point, we only know about a few different content packs, and there's a deal for a free Defense of Crete Booster Pack when you buy both Premium Civs, so everything on the market right now will set you back only about $60. But keep in mind that the Season Pass lasts until February of next year, so everything Gas Powered releases before then is yours to keep. If you like the game a lot, that will probably be worth the deal.
One note of warning: Gas Powered also says that buying the Season Pass won't keep you from rebuying content accidentally, so don't forget and buy more stuff when your Christmas cards come in. But hopefully if you spend $100 on a free-to-play game, you won't really be tempted to spend any more than that, right?