Question

So Alex is selling a very rare spice that costs $100 per gram. Someone wants to buy $80 worth of the spice. How much spice (in grams) does Alex give him?

Answer 1

80% of one gram is 0.8 grams. It's literally that simple.

Answer 2

80% of a gram.
One gram is $100, aka $100 is 100% of a gram, so $80 would be 80% of a gram.