Count the costs, but don't count the benefits.
Academic studies of the economic impact of illegal immigration into the US find that it is at worst a net neutral for the economy. The vast majority of studies, by non-partisan, right and left leaning groups find a net benefit to the US economy.
Even conservative organisations like the CATO Institute, American Enterprise Institute or Heritage Foundation find that there is a net economic benefit to immigrants - even for illegal immigrants. (CATO just released a 'UGE white paper on the topic:
https://www.cato.org/white-paper/fiscal-impact-immigration-united-states#executive-summary.)
You have to get into REALLY dubious sources like the Center for Immigration Studies or FAIR before you start getting into territory where illegal immigration is a cost to the economy. And, it is childishly simple to look at their methodologies and work out that they're largely working backwards to arrive at presupposed conclusion. To quote the CATO Institute, such studies "vastly overstates the costs of illegal immigration and totally ignores the benefits."
The main negative impact from illegal immigration appears to be on the wages of males without a high-school diploma in blue collar/industrial roles. Competition from illegal immigration has reduced their wages by an estimated 4% to 12%, depending on the study.
Meanwhile, the positive economic impacts of granting citizenship to 8-11 million undocumented migrants has been estimated by left leaning sources (including the current White House) at between $100 billion and $170 billion per year and by more conservative sources at $58 to $115 billion.
Another study pegs migrants as having a net positive impact on federal taxes of around $9 billion and for state taxes of around $7 billion. Yet another puts illegal immigrants contribution to federal taxes at more than $11 billion and for state and local taxes at nearly $12 billion.
$5 billion wouldn't have fixed the problem.
In 2016 MIT priced a partial (1000 mile) border wall at $38-40 billion
Initial DHS projections in 2017 were $21.6 billion over nearly 4 years for 1235 miles (a cost of $17.5 million per mile), plus ongoing costs of more than $1.7 billion per year once completed. Follow up DHS projections in 2020 were $18 billion for construction of just 722 miles (about $25 million per milt).
Trump White House estimates in 2018 were that construction of a southern border system required $25 billion, at a cost of about $33 million per mile.
The CATO Institute projected in 2019 that the cost would be $59.8 billion for 1637 miles, based on numbers from the Office of Budget Management. They also estimated that it would cost nearly $865,000 per mile to man and maintain such a fence - or more than $1.6 billion per year.
Keep up that fear mongering. Gold star for you!