According to the announcement, the expansion will be available on both Windows, Macintosh on DVD as well as digital download at 39.99 USD. The collector's edition would be available at 79.99 USD. Given the high sales volume in the previous expansions, Cataclysm is expected to turn out similarly.
After lots of speculation and anticipation regarding when the expansion would be released, Blizzard has recently made an official announcement that the expansion would go live at the end of this year. At the moment, the beta test is being carried out. To participate, you would need to sign up for a Battle.net account. Check out more information about the release here.