Add the following script to the large or small products layout for an online store:
<script language="javascript"><!--
var onsale_{tag_productid} = "{tag_onsale}";
if (onsale_{tag_productid} == "0") {
document.getElementById("rrpprice_{tag_productid}").style.display = 'none';
}
//--></script>
Then you would assign the following tag ID to a retail price tag
<div id="rrpprice_{tag_productid}">{tag_retailprice}</div>
After adding this script, the retail price will be hidden for every item that is not currently on sale.