just because it's bigger don't mean it should cost more, what its made of is what causes the price. Otherwise they could just make a full-tower out of 100% plastic and sell it for 200bucks and that would be ok?
Of course it should! The extra metals for base structure dosen't come for free you know
And you're forgeting that plastic aren't cheap too, plus the paint job, molding, labour, etc. If NZXT were to make a $200 plastic case then Corsair would make a $700 metal case..
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