The budget on the wall is worse than the bottom line figure because normally when a government spends 30 billion it is on an asset that is productive and pays dividends: highways, science, medical research, dams, nuclear power stations, universities etc.
But a wall is absolutely unproducfive, the majority of the people it may deter would have paid taxes and work for low wages adding to the economy so even if you want to put a dollar figure on keeping people out, it is a nett negative. Spending 30 billion on a wall is like 300 billion not spent on infrastructure projects