I have gotten up to around $4750 per tap. I have a neighbor eeno (who shall remain nameless) with 120 burning bushes at .5%, 11 news vans at 1%, 6 world broadcast news dishes at .75%, 40 orange trees at .5%, and the Springfield sign of course at 4.5%.
That is roughly 100% bonus each tap without the benefit of the other premium items that I failed to mention.
My game usually doesn't crash when visiting friends so it has been fairly easy to collect $1M+ each day.
This neighbor doesn't even play anymore, as there is no hospital or Hibbert office or ambulances.
I just lucked out I guess.