What I've noticed is that arrowheads, broadheads and fish hooks all suffer from same cost estimate failure. The Smith gives you an cost estimate of a single head/hook, not the whole batch.
You can also purchase a single item from the smith too: When asking if the order is ready and smith confirms, hit
0 to return to chat top menu*, then
trade, and you can select how many of the heads/hooks you actually want to barter for.
*the smith will nag about "you not better leave him with the ordered goods", don't mind the grouch, it's just lack of fresh air