Create a new variable in database products_def Create a new field in admin products.tpl Paste your variable into pages_default.tpl and products_default.tpl Use CSS to make your old price line-through or alike
4. and don't forget to paste this new variable $aData[mOldPrice] somewhere in templates/products_defaul.tpl too. For example somewhere in <!-- BEGIN PRICE --> block.
You should upgrade it if you wanna get it well done. It's just a simple sample.