Catalog Price Rules - Action - By Percentage of PURCHASE PRICE

I want the wholesale price to be calculated in procent of my own purchase price, not the customer price (why would anyone want that anyway?).

If my phurchase price (from manufacturor) is $100 (ex vat), I want the wholesale price to be let's say 110% of 100 = $110.

But there is only a "By Percentage of original price" option in the Cathaog Price Rules - Action.

Now there is no attribute called "phurchase price" in Magento, but what if I make one? Can I use that attribute to calculate the prices for the Wholesales Group? Does the code below has something to do with it? Can I change that somehow so it work the way I want?

Quote:

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The first setting is a drop-down list called Apply. It has four options, which are used by the Mage_CatalogRule module’s helper class to calculate the price:

public function calcPriceRule($actionOperator, $ruleAmount, $price)
{
$priceRule = 0;
switch ($actionOperator) {
case 'to_fixed':
$priceRule = min($ruleAmount, $price);
break;
case 'to_percent':
$priceRule = $price * $ruleAmount / 100;
break;
case 'by_fixed':
$priceRule = max(0, $price - $ruleAmount);
break;
case 'by_percent':
$priceRule = $price * (1 - $ruleAmount / 100);
break;
}
return $priceRule;
}

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