Use non-union labor. Cost$ will plummet 50%. If only New Mexico and California were Right to Work States this would be possible.
Being a Federal Project I’m not sure they could compel the Federal Government to employ only union labor workers.
“Use non-union labor.”
Lots of ways to reduce the labor costs. Offer non-violent convicts a chance to earn reductions in their sentences for volunteering their labor. Or, make it a CCC-type jobs program - earn some money and learn a trade. These are just a couple of suggestions. I’m sure there are a lot more. People who are out of work because they lost their jobs to illegals would probably jump at the chance to put a stop to this epidemic of off-the-books labor by helping build the wall.
NM and CA are small pieces of border compared to other states.