Sonic Adventure, the Dreamcast classic, is going to be making a debut on XBLA on Sept 15th for 800 MS Points.
Revamped in HD, this Sega classic will play exactly how you remember it. Sega is also planning on releasing Crazy Taxi on XBLA but no further details are currently know. But if Sonic Adventure is priced at 800 MS Points ($10), then this would probably be a safe price assumption for future Dreamcast ports.