[JWait]

You could try including the routine from products.tpl that determines whether to show the "market price" or not...

{if $products[product].taxed_price ne 0 or ($products[product].variantid && $products[product].variants_has_price)}
{if $products[product].list_price gt 0 and $products[product].taxed_price lt $products[product].list_price}
{math equation="100-(price/lprice)*100" price=$products[product].taxed_price lprice=$products[product].list_price format="%3.0f" assign=discount}
{if $discount gt 0}
&nbsp;<font class="MarketPrice">{$lng.lbl_market_price}: <s>
{include file="currency.tpl" value=$products[product].list_price}
</s></font>
{/if}
{/if}
{/if}

You may only need the part that begins at {if $discount gt 0} to the first {/if}. This isn't going to total the market price though in the even a customer buys more than one item.

I don't know if this will work or not, but it is someplace to start.