$18.50 is a steal (I’ve heard of $22/lb)
As far as how much, what we buy is 2 lbs per person. The usage plummets after day 1 and 2, and if you are lucky enough to run out, you can always buy more on Chol Hamoed (or get from your brother-in-law, who overbought by 12 lbs.. just in case)