If we cut the salaries of congressmen and other federal offices, we'd get some of the funds. We then move to state and city level and cut salaries there. Next, we'd reform the tax system and make it a flat percentage across the board, but with no charity loopholes if you're worth above one million. That would force them to pay more. After that, we'd put some serious reforms on the well-fare system, possibly getting rid of it altogether. Last of all, we'd pull all our troops from the Middle East and stop looking for oil in places we don't own.
It shouldn't be that expensive of a project since we'll be connecting pieces of wall that are already there. We can use the leftover money to get a little out of the national debt.