Don't know if this was mentioned, but swapping border walls when you collect after backlogging taxes and not hanging on to them for bill time mitigates the effect of any upkeep costs.
Your sign is reversed for the 5th Border Wall increase, so the total 5-Wall increase is $67814/day. Over a 10 day collection cycle, $600k is nothing to sneeze at. Granted, when you net $50M in the same span, a 1.2% income increase probably won't be of that much help after all. Anything to stay awake when you're in the top 5%, eh?