A budget is an estimate of spending and income, but once it was a satchel. The word comes from the French for a small leather bag, and was first used in English in the 15th century. Travelling pedlars carried their wares in a 'budget', but later it was used for paperwork and the word came to mean the contents as much as the bag.
A letter full of news could also be a 'budget'. In the 18th century, the Chancellor of the Exchequer began to 'open the Budget', revealing his financial plans for the year and that bag of tricks gave its name to all other 'budgets' ever since.