title: Represent numeric value with typical dollar amount format
body: <p>I have a data frame storing the dollar amount, it looks like this</p>
  
  <pre><code>&gt; a
    cost
  1 1e+05
  2 2e+05
  </code></pre>
  
  <p>I would like it can be shown as this</p>
  
  <pre><code>&gt; a  
    cost
  1 $100,000
  2 $200,000
  </code></pre>
  
  <p>How to do that in R?</p>