If you want the Mythic II Map Pack, that's going to cost, but not nearly as much if you're a Gold customer.
Halo 3, the game that transformed the franchise from something known only to Halo fans to a mass market phenomenon, is free to download for Xbox Live Gold customers, this month only. The promotion also includes the Mythic II Map pack, but that will cost a small amount of money. Gold subs can get the Mythic pack for $4.99; it costs $9.99 otherwise.
This one did crazy business back in 2007 and, once upon a time, Microsoft claimed it outsold all of the PS3's heavy hitters combined - titles like Resistance 1 and 2, Uncharted 1 and 2, Killzone 2 and God of War III - while its Believe ad made the 25 "most epic" list. Meanwhile Covenant are still dying in their billions; why not go through another few hundred this month?
The promotion ends October 31st.