As Priscilla said, it's a slight distinction. To me, it seems that "by now" is more interested in the present state : "it should have been done by now" = it doesn't matter if it happened just 10 seconds ago, as long as it is done Whereas "before now" is more interested in the time when the thing should have been done: "it should have been done before now" = it is as if you just did this now where it should have been done before I hope it clarifies your question!

The difference is very small, but "by now" refers to anything that has happened previously up to this moment. "Before now" means further back in the past. Example: She should have gotten here by now. (Up to this moment.) Example: We should have reviewed the documents before now. (We should have reviewed the documents earlier.) I hope this helps. The difference is subtle and I wouldnt worry too much about using it incorrectly.