I couldn't find all the necessary data for FY18, so this is for FY17. Source:
https://www.cbo.gov/publication/53625 Total revenue: $3.3 Trillion.
Total spending: $4 Trillion.
Resulting deficit. $700 Billion, or 17.5%
Of that $4 trillion spent, only $1.2 trillion was discretionary. The mandatory spending was mostly Medicare/Medicaid, Social Security, government retirement pensions, income security programs (tax credits, SNAP, unemployment, and the like), and VA benefits. Note that actual military spending is *not* included in mandatory spending.
So what does this mean? Even if congress were to say "cut out all discretionary spending. No military, no parks, no NASA, no FBI, no Justice Department. Nothing." We would still be spending 85% of our budget.
The kicker? Federal debt is at almost $22 Trillion. So even with 0% discretionary spending, spending 85% of our budget would have us paying off our debt in approximately... 44 years, assuming *no* interest. Not necessarily saying we should pay off our debt, but just as a point of how much our debt is in relation to our spending and income.
So yes, let's worry very much about how much we spend on border security. Let's wrangle over a few billion here and there. A millionaire is made ten bucks at a time, right? We might shave a few days off how long it takes the good ol' US of A to reach FI.