Currently, Famitsu pinpoints the downloadable costume content at 320 each, or you can wait for the entire "combo pack" for 1,000 after all the costumes packs have been released. Note that this content has only been confirmed in Japan. We are waiting on a response from Capcom.
Here's the official press release from Capcom announcing and detailing the additional content for America as well.
Capcom is pleased to announce the first expansion pack for Street Fighter IV. Championship Mode expansion pack will add several unique and gameplay enriching options to Street Fighter IV's already robust feature set free of charge post launch. Street Fighter IV's Championship Mode Expansion Pack will include:
-- Replay Mode - Record, upload, and download top matches to share with the Street Fighter IV community. Players can analyze top tiered fighters matches, leave voter feedback for others, and share their own triumphant victories.
-- New Points System - Two new points systems to measure your skill (Championship and Tournament Points)