So how much does a teaspoon of mayonnaise cost anyway? Is it comparable to the prices at your local sandwich shop? Does it cost less, or is it possible that it is actually more expensive to use your own ingredients? If you have a few minutes and a record of what you paid for your ingredients, you can calculate the cost per serving.

Write down the total cost of the ingredient that you purchased, adding tax, if applicable. This number represents the total cost that you paid for your ingredient.

Determine how much of the ingredient will be used in your recipe, and write down that number as a measurement. For instance, it may be one egg, or one ounce of oil or one cup of milk.

Convert the measurement into a fraction, with the "quantity used" on the top half of the fraction, and the total quantity purchased on the bottom of the fraction. For instance, a recipe that called for one egg would require that you use 1/12th of a dozen.

Divide the total cost of the ingredient by the total quantity of goods, which is recorded at the bottom of your fraction. For instance, a dozen eggs that cost 70p, would have a unit cost of 5.83p per egg.

Multiply the amount of the ingredient used in your recipe by the total cost per unit. Using the above examples, with a recipe that called for one egg, which was 1/12th of a dozen. The total cost of goods was 70p, or 5.83p per egg. If the recipe called for only one egg, the cost for that ingredient is 5.83p.

Repeat this process for every ingredient, writing down cost on a piece of paper. Add the cost of each ingredient together, and you will have the total cost for your dish.