With this module you can show double prices for all product, based on regular price ($price). Just write your percent and name your Price_2 in watever you want.
You can set currency sign in front (example: $10) or behind price (example: 10E). Module show double price only on product page.
Installation: Change your percent, name of Price2 and currency -> upload in xml VQMOD folder or OCMOD for 2.x

THIS MODULE IS MADE ONLY FOR 1 LANGUAGE AND 1 CURRENCY !

DEMO: http://c-bit.hr/racunala/stolna-racunala?product_id=7560

This module not affect checkout at all, but you can use it in combination with this module:
http://www.opencart.com/index.php?route=extension/extension/info&extension_id=704
or
http://www.opencart.com/index.php?route=extension/extension/info&extension_id=14078
or
http://www.opencart.com/index.php?route=extension/extension/info&extension_id=4099