"...And then I found five dollars" is a phrase Young Hip People use to end a boring or irrelevant story. Ideally, the phrase is used during an anecdote that seemed exciting before it was shared, but was actually boring, mundane, or irrelevant. (Happens to me all the time.)
The phrase is used to make the story more interesting, provide an ending quickly, and acknowledge the lack of excitement in the anecdote.
The phrase is best said with excessive exuberance or almost monotone. It's important that your tone of voice imply that you did not find five dollars and are acknowledging both the end of your boring storing and its irrelevancy.
It's possible that your anecdote is fascinating and worthy, but is ill-received. In the event that it happens to you (and it's far more likely—your stories are always great), you may use this colloquialism. In this case, you're acknowledging that your audience is bored and are alleviating an unpleasant response.
And then I found five dollars, Urban Dictionary