3DS Max is expensive because it's a professional tool from Autodesk. Pretty much every modable game requires some form of some Autodesk modelling program (be it 3ds Max, Maya, or XSI)... maybe Lightwave though they're still independent -- at least as independent as you can consider NewTek.
So yeah, the reason why it's so expensive is simply because it is. You just cannot avoid it... there's no way Bungie is making their own editor for modders if they didn't even make it for themselves. Bungie themselves use 3D Studio Max (and a bit of Maya).
As for why include the editor at all if no-one can use it properly? If you're talking about not having 3D Studio Max? Well... I bought a copy of Maya to do mods for Unreal Tournament 3. If you're talking about the editor just plain being broken in how crippled tool and the tag library is? Well... then you have the reason why I didn't use Halo 2 to mod.
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