Gang violence in Central America drives much of the immigration across our southern border, so let’s invest in proven programs that can reduce that violence. A much-studied program called Cure Violence achieved 45 percent reductions in violent crime in Trinidad and Tobago and up to 90 percent reductions in shootings in Honduras.

Additional anti-gang initiatives in Central America could save lives, reduce the “push factors” that drive migrants and reduce the need for border security — all at a much lower cost than a wall.

Drug overdoses kill far more Americans (70,000 a year) than immigrants, terrorists, guns or automobiles, yet only 10 percent of Americans with addictions get specialized treatment. So one way to save American lives and also reduce the crime that often goes with addiction would be to provide treatment that helps people break drug dependency.

One of the most successful programs, Women in Recovery, works with moms in Oklahoma who are drug offenders; for each participant, it costs $28,000, lasts 18 months, ends with the woman employed and has only a 4 percent recidivism rate over three years. The children also benefit as their mothers move from addiction to regular jobs — so let’s expand it nationwide.

More than half a million American children still are poisoned with lead each year, damaging their brains and increasing the risk that they will become criminals. One study found that a large-scale effort to reduce lead poisoning would save the United States billions of dollars in costs of crime and special education. Likewise, job training for prisoners through the Center for Employment Opportunities costs $6,500 per person and has a good record of turning offenders into taxpayers.

Research suggests a higher return from investing in education than in walls; early childhood, when the brain is developing, is a particularly crucial window. Home visiting programs like Nurse Family Partnership coach at-risk moms and have an excellent record.

Another high-return investment: provide girls who don’t want to get pregnant with long-acting contraception such as IUDs and implants.

Wow! We’re spending our “wall money” so cost-effectively that we have a chance to reserve some for humanitarian causes abroad. For example, millions of people go blind from cataracts, which a five-minute surgery can fix for $50. Similarly, 40 cents will deworm a child for a year, resulting in a kid who is healthier and better able to stay in school. Or vitamin A supplementation reduces the risk that a child in a poor country will die or go blind.

Even after all that, I have money left over. So let’s get creative.

The G.O.P. has sharply cut funding for the I.R.S., so that the audit risk has plunged for zillionaires. The Justice Department estimates that each additional $1 spent on tax compliance brings in an additional $6 in tax revenue. So with a modest investment, we can start all over again spending money!

If we start a fund to invest in social-impact bonds and performance-based contracts, we can tap private investors to address America’s needs — and pay only for success.

Finally, in a bit of whimsy: experiment with dog subsidies. True, this has not been subjected to randomized trials (any more than the wall). But mental health is underaddressed in America, with suicides at a 30-year high, and what greater comfort is there than a friendly dog?

So I challenge you. How would you spend the $5.7 billion to make America better off? You can use all or part of the money to begin to build a wall (credible estimates for the cost of the full wall start at $22 billion), or you can allocate it to whatever area you believe generates the most bang for the buck. Then you can post your results on social media.