Usually not that many. If Trump had ignored the ad, they probably would have no impact outside of Iowa. But Trump has now made them a national story. Every person who hears about the lawsuit will also hear about the ads that prompted threat. It will make a lot of people who would have never seen the ads otherwise go and look to see what all the hoopla is about.
You claim Trump will get $50 million in free publicity about the suit - he will also give $50 million in free publicity to the ads. Can't have one without the other.
We can debate this until blue in the face, with no end result. As they say, the test of a pudding is in the taste.
When January 2016 rolls around, and primaries begin, we can then watch, if Trump is still winning by comfortable margins. That will be the test to decide if Club for Growth won or Trump prevailed in spite of arrows shot by rivals.