The US government does not have a revenue problem, they have a spending problem. Because they treat the national debt separately from other spending, whether it be discretionary or not, they spend and spend like crazy. Then they remember they have to borrow more to pay the debt. There is enough revenue taking in by the government that should easily pay normal obligations. It will never change until they are forced to have a balanced budget to include paying the debt.
From personal experience, there are those times when revenue does not equal outflow, but the outflow can be cut or revenue increased by making an investment, even if that is done on credit.
In the future, through savings, that investment will pay for itself. Some see that as a more reliable vehicle, for some it is the purchase of tools to fix things which would cost far more, A student loan is a common example to obtain the qualifications for a better job or higher pay.
A means, including a physical barrier, of controlling the influx of people who cost the taxpayer in services, criminal activity, drug trafficking, and a host of other ways, is an investment in America's future.
So, too, is the restoration of functionality, not just to the industry of flood affected areas, but to the daily lives of the millions of people who have been affected by this flood, and the restoration of the economic dynamics involved in such a large community.
It isn't a question of which can be paid for, we cannot afford to shirk either.
If cuts must be made elsewhere, so be it.