"... The dollar is in sorry shape tonight;
What the dollar used to get us
Now won't buy a head of lettuce."

may be meant to mean: nowadays a dollar does not even buy a head of lettuce - not related to income, just inflation. If you wish to change it to be about income, it must run like "what a day's work nowadays gets us ...", which rhymes with lettsuce.

In many cases, making a new song is the best solution. Still, I am looking forward to reading from other "improvers" in this thread.