Any input element have subscribed to the changes of dropdown list, it's value will be set to the selected option's [data-publish-value] when the dropdown list changed.
Download the production version or the development version.
In your web page:
<script src="jquery.js"></script>
<script src="dist/jquery.dropdown-subscriber.min.js"></script>
<select id="dropdown" name="dropdown">
<option value="">Please Select</option>
<option value="1" data-publish-value="10" selected="selected">A</option>
<option value="2" data-publish-value="20">B</option>
<option value="3" data-publish-value="30">C</option>
</select>
<input type="text" id="input" name="input" value="10"
data-dropdown-subscriber="#dropdown" />