Comment out this piece of code in /modules/Product_Options/customer_options.tpl:
Code:
{if $v.is_modifier eq 'Y' && $o.price_modifier ne 0} ({if $o.modifier_type ne '%'}{include file="currency.tpl" value=$o.price_modifier display_sign=1}{else}{$o.price_modifier}%{/if}){/if}
Note that this is a global change. If you want other products in your shop to show price or % in the options, you will have to write specific {if} statements.