title: Product base price multiply with custom option
body: <p>we would like to sell pre-paid gift cards which is our customers should specify the amount of cart.</p>
  
  <p>So, I have created a custom option to specify card balance then I would like to multiply with card quantity. </p>
  
  <pre><code>CARD A
    Balance = 50$
    Quantity = 10
  Total = 500$
  </code></pre>
  
  <p>So my question is, how can I do this approach?</p>
  
  <p>Thanks,</p>