You get three Fastpasses a day, split into tiers (eg 1 x E-ticket, 1 x B-ticket, and 1 x C-ticket).
At the moment there's no upcharge for rides as such, but it seems like for the future they're looking into letting you pay extra to have more than the base amount of Fastpasses, and if attractions go Fastpass only, then those who have already used up their Fastpass for a specific attraction's tier will be able to purchase a ride on another attraction in that tier for an additional upcharge.
I suspect the end game we're heading to is the following: Your base cost might be $105 to get into the park, including three Faspasses you may use for Big Thunder, Buzz and 7 Dwarfs. You then pay an extra $5 for a compulsory Fastpass to Space Mountain, $5 for Splash Mountain, and a $10 package that gives you all the Fantasyland dark rides. Possibly everything else (Carpets, Treehouse etc.) will still have regular standby lines.
Even if you decide to ride Space twice, you're only paying $25 dollars extra over regular admission but for that price will never wait in a very long line as you will always have a Fastpass, or be going to a near-enough walk-on, so you will have lots more time for shopping and dining. That's also a lot cheaper than an Express Pass at Universal too, which provides similar benefits.