I believe only the Price Modifier shows the amount added by the option (not variants).
Variants can be tricky. If there is just one variant option then in the same file totaltec mentioned the following code adds the total price for the variants to the drop-down menu. Before </option>:
PHP Code:
{foreach from=$variants item=vv}
{foreach from=$vv.options key=vvk item=vvo}
{if $o.optionid eq $vvk}
{include file="currency.tpl" value=$vv.taxed_price plain_text_message=1}
{/if}
{/foreach}
{/foreach}