Looking at the source of the actual page, it's very odd markup.

There's a real input field, and a hidden input field "on top of" each other. I guess one is the stylized user-interactive one of which the value gets normalized into cents into the hidden one?

   <span class="variable-amount">
   $
   <input class="variable-amount-input" type="text" style="width: 25px;">
   <input name="amount" type="hidden" value="19900">
   /year
   </span>