How do you calculate the estimated cost of a budget?

You generally use historical pricing. If you've been tracking your spending for a few months, you use that history to estimate the costs in developing your budget.

If you don't have a history, you might get some input from your friends and neighbors about what they pay for utilities, phone, food etc.

You can always adjust your estimates as you develop a history of expenses.

There was me thinking the budget was the amount of money you needed to stay within when working out the costs of everything... Unless you're gathering up that money and want to estimate how long that might take. Since cost can relate to resources, time and the literal money involved.