Except that $40 cassettes double well as boat anchors. Bleh...
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True to some degree though the weight difference between a $40 cassete and a $100 is pretty minimal. It's only 60 grams between the $100 xt and the SRAM pg970 at $45.
I don't worry about it cause I run just one cog back there. But have run the cheaper SRAM pg cassette for a year on my other bike with no issues.