<config>
   ...
   <global>
      ...
      <catalog>
         <product>
            <type>
               <simple translate="label" module="catalog">
                  <price_model><!-- YOUR_PRICE_MODEL --></price_model>
               </simple>
            </type>
         </product>
      </catalog>
   </global>
</config>

class /*YOUR_PRICE_MODEL*/ extends Mage_Catalog_Model_Product_Type_Price
{
   public function getPrice($product){
      // your custom calculate price
   }
   public function getFinalPrice($qty=null, $product){
      // your custom calculate final price
   }
}