Here's what I use in modules/Product_Options/customer_options.tpl:
Code:
<OPTION value="{$o.optionid}"{if $o.selected eq 'Y'} selected{/if}>{$o.option_name}{if $v.is_modifier eq 'Y'} ({if $o.modifier_type ne '%'}{math equation="price+modifier" price=$product.taxed_price modifier=$o.price_modifier assign="optionprice"}{include file="currency.tpl" value=$optionprice}{else}{$o.price_modifier}%{/if}){/if}</OPTION>