They did it on purpose.
They were counting on people to buy motes instead of heavy because "motes rank up faster" if you don't take the 10 seconds or so to do the math.
English
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Buying motes from Xur is a robbery
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Worse than the instant waste of marks if you accidentally slip and buy a class item from the Speaker instead of a material in my opinion. I'll admit I bought quite a few motes then I decided to stop and see what the real exchange rate was and I was pissed as hell.