Needed this feature too for a client and forked the core with a very straight forward solution.
1 . Replaced the Public function getPrice for this one
* Default action to get price of product
* @return decimal
public function getPrice($product)
$product = Mage::getModel(\\\'catalog/product\\\')->load($product->getId());
return $product->getData(\\\'cost\\\') * $product->getMarkup();
2. Placed the new file in local… app/code/local/Mage/Catalog/Model/Product/Type/Price.php
3. Eliminated the Product atribute Price in every Attribute Sets (not necessary, just for my client do not mistake)
4. Populated the cost attribute with the prosuct costs, and changed to Required
5. Created a new one and added to all Attribute sets, named Markup (code \\\"markup\\\") and set to required
Now the store capture the cost and multiply the markup.