calculate VAT on cost when saving products and apply at price

Question

i am new to Magento.i am trying to achieve a simple way to apply custom margin at cost and then get its sale price.. i want to put only cost then some function to calculate 20% VAT (TAX) and custom attribute... and set margin 15%

example;

i have Three fields..when adding or editing product

 Cost = 100 
 Price = 0 
 margin_percentage: 15 (means 15%) 

after user add 100 and margin percentage 15, system first calculate 20% vat then on resulting amount add 15% margin so i this case

cost + vat + margin = 138 after saving

 Cost = 100 
 Price = 138 
 margin_percentage: 15

i have write a module and trying to achieve this using Event Observer - catalog_product_save_before and catalog_product_save_after here is a function ...if this code works i may apply formula’s ..

  class SmashingMagazine_LogProductUpdate_Model_Observer
  {

  public function logUpdate(Varien_Event_Observer $observer)
  {
    $product = $observer->getEvent()->getProduct();
    $cost = $product->getCost();

    $_product = Mage::getModel('catalog/product')->load($product->getId());
    $_product->setPrice($cost); // or an other price field
    $_product->save();

    Mage::log(
        "cost {$cost} Price {$price} updated",
        null, 
        'product-updates.log'
    );
  }
  }

when i create or edit product at catalog... above code not working and product price page like some infinite loop.. at end timeout and values not saved. please suggest me some hints and i am very confused how I accomplish this task. i have no experience of Magento research a lot for the last 5 days. i am following this tutorial to make module http://coding.smashingmagazine.com/2012/03/01/basics-creating-magento-module/

Answer 1

The loop is created because you do a save for the product. If you want to change attributes than you use the before save observer, since the object us saved after your xhabge you do not need to save it yourself.