She started her business 'on a small/tight/shoestring budget' [=she did not have much money when she started her business] and could not afford to overspend.
The government may have to raise taxes to 'balance the budget'. [=to have enough money to provide the amount that is being spent]
The project was completed on schedule and 'under budget'. [=it was completed for less money than had been planned]
(..) Give it up for Ned. You can start the morning in Boston, stop in Dallas over lunch, and finish the day in San Francisco, all for the price of a T line. Our inflated travel budget is eviscerated by %. And more importantly to you guys on the road, no more Christmases in a hotel in Tulsa, (..)2009 Up in the Air