I remember when I was in college and first started shopping for my own groceries, that I would often compare the cost of restaurant dining to the grocery store prices for food. Now I do the opposite. I justify a $5 marinade when I figure the cost of it plus chicken is still less than those ingredients in a restaurant. That's messed up.
To put your example into perspective: that $5 marinade is enough for about 8 chicken breasts so that's only 60 cents per piece of chicken. That's completely reasonable. Charging $18 for a meal consisting of a chicken breast, a side of mashed potatoes, and a veggie is not as justifiable.