“...there should have been a high tax (at least 20-25%) imposed on all remittances sent to Mexico by relatives of Mexican nationals.”
That has been an oft-quoted formula (with varying percentages) for wall funding. An interesting question would be: at what percentage would the law of diminishing returns compel remitters to give up earning money here and move back there? Also, is the remittance charge/tax better as a wall-funding solution or as a mechanism of self-deportation and a disincentive to illegal immigration?
It would have both functions. It would create a disincentive for some to come here to begin with, an incentive to leave for some already here, and a source of tax revenue from those who remain. I'm not sure what the optimal level of taxation would be to balance the last with the first two. Around 25% isn't unreasonable, as that's lower than what some income brackets pay in income taxes.