What you're paying extra for with 99designs is a better rank of designer. That's not to say the designers on DesignCrowd are bad—far from it, many are noticeably quite talented—but both the size of 99designs' community and its process for sorting its members delivers better quality designers with more consistency.
How much can I earn? 99designs is free to sign-up and start freelancing. Rates and rewards vary depending on the type of design work. You'll be paid in three business days (or less once you become a Top Level designer).