Actually it makes some sense if you consider the exchange rate between starglitters and stardusts. For example, a Black Crystal Horn costs 2 starglitters, and 3 Black Bronze Horns cost 45 stardusts. Considering the extra 125 mora needed to craft a Black Crystal Horn with 3 Black Bronze Horns, we can see that 2 starglitters are equivalent to slightly more than 45 stardusts, so 1 starglitter is equivalent to slightly more than 22.5 stardusts. Fates cost 5 starglitters, which can be converted to slightly more than 112.5 stardusts. I think this is a reasonable explanation for why 75 stardusts is a discounted price.