Unless you’re pricing the cost of institutional (that is “high volume”) food production, you’re not going to be using salt by the kilogram in any domestic cooking. So something that’s as cheap as salt is, in the quantities you’re likely to use it in any home cooking recipes, is going to be “lost in the noise”. That is, the slight variation in price from place to place and brand to brand of “salt” (a minor ingredient, for the most part) is going to be figured in pennies per serving.
Look to the actual prices of your main (and costly) ingredients, and then just throw in an estimated value for the cheap ones, and the cheap ones that are used in small quantities… like salt.
Welcome to Fluther, by the way.