Good things come to those who wait, right? Next week, on March 25 at 2 a.m. Pacific time, Bungie's Heroic Map Pack will be made available free of charge to all Live members. The series of maps were originally released last December for 800 Microsoft Points.
Also on Tuesday, Bungie will officially reveal the third map in its upcoming Legendary Map Pack, which is due out on April 15. Unfortunately, Microsoft and Bungie have no present plans to make this map pack free in the foreseeable future.
Bungie Weekly Update [Bungie]