actually now i'm not sure. but i know at least 3 of them will come with Halo Wars LE when it comes out mid March.
CORRECTION: 3 for sure, i just checked. assembly, orbital, and sandbox. and they won't come out on XBL marketplace until after Halo Wars LE comes out. which will be $80,000 and include the maps, a graphic novel, a couple of in-game bonuses and some patch thingy.