<fields>
           <goldprice translate="label">
               <label>GoldPrice</label>
               <frontend_type>text</frontend_type>
               <sort_order>0</sort_order>
               <show_in_default>1</show_in_default>
               <show_in_website>1</show_in_website>
               <show_in_store>1</show_in_store>
               <comment>Gold Price per 1gm</comment>
          </goldprice>
          <run translate="label">
             <frontend_type>button</frontend_type>
<frontend_model>letsnurture_customgrid/adminhtml_system_config_customfield</frontend_model>
             <sort_order>30</sort_order>
             <show_in_default>1</show_in_default>
             <show_in_website>1</show_in_website>
             <show_in_store>1</show_in_store>
        </run>
</fields>

<?xml version="1.0"?>
<config>
  <modules>
    <LetsNurture_CustomGrid>
      <version>0.1.0</version>
    </LetsNurture_CustomGrid>
  </modules>
  <global>
    <blocks>
      <customgrid>
              <class>LetsNurture_CustomGrid_Block</class>
      </customgrid>
    </blocks>
    <models>
         <class>LetsNurture_CustomGrid_Model</class>
    </models>
    <helpers>
      <customgrid>
        <class>LetsNurture_CustomGrid_Helper</class>
      </customgrid>
    </helpers>
    <events>
          <admin_system_config_changed_section_gold> <!-- identifier of the event we want to catch -->
          <observers>
              <customgrid>
                  <type>model</type>
                  <class>LetsNurture_CustomGrid_Model_Observer</class>
                  <method>UpdatePrice</method>
              </customgrid>
          </observers>
       </admin_system_config_changed_section_gold>
  </events>
  </global>
</config>

class LetsNurture_CustomGrid_Block_Adminhtml_System_Config_Customfield extends Mage_Adminhtml_Block_System_Config_Form_Field
{
    protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element)
    {
        $url = $this->getUrl('catalog/product'); //

        $html = $this->getLayout()->createBlock('adminhtml/widget_button')
                    ->setType('button')
                    ->setClass('scalable')
                    ->setLabel('Run Now !')
                    ->setOnClick("setLocation('$url')")
                    ->toHtml();


        return $html;
    }
}

<frontend_model>NAMESPACE_MODULENAME/adminhtml_system_config_customfield</frontend_model>

class NAMESPACE_MODULENAME_Block_Adminhtml_System_Config_Customfield extends Mage_Adminhtml_Block_System_Config_Form_Field
{


    protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element)
    {
        $html = parent::_getElementHtml($element);
       //Write your custom html here
        return $html;
    }
}

<crontab>
    <jobs>
        <sariina_payvast_purchase_price_update>
            <schedule>
                <cron_expr>* 1 * * *</cron_expr>
            </schedule>
            <run>
                <model>sariina_payvast/observer::getMerchandiseLastBuyRate</model>
            </run>
        </sariina_payvast_purchase_price_update>
        <sariina_payvast_stock_update>
            <schedule>
                <cron_expr>30 1 * * *</cron_expr>
            </schedule>
            <run>
                <model>sariina_payvast/observer::getMerchandiseRemainForAllStockRooms</model>
            </run>
        </sariina_payvast_stock_update>
    </jobs>
</crontab>

public function getMerchandiseLastBuyRate()
{
    if (Mage::getStoreConfig('payvast/general/enable_purchase_price'))
    {
        Mage::log('Payvast LastBuyRate Update Start...', null, 'payvast.log');

        $client = new SoapClient(Mage::getStoreConfig('payvast/general/url'));
        $products = Mage::getModel('catalog/product')->getCollection();
        $products->addAttributeToSelect('sku');
        $products->addAttributeToSelect('last_purchase_price');
        foreach($products as $product)
        {
            Mage::log('Updating Last Purchase Price for '.$product->getSku(), null, 'payvast.log');
            $params = array(
                'merchandiseCode'   =>  $product->getSku()
            );
            $response   =   $client->GetMerchandiseLastBuyRate($params);
    //Mage::log($response, null, 'payvast.log');
            if ($product->getLastPurchasePrice() != $response->GetMerchandiseLastBuyRateResult/10)
            {
      /* Rial to Toman */
                $product->setLastPurchasePrice($response->GetMerchandiseLastBuyRateResult/10)->save();
            }
        }

        Mage::log('Payvast LastBuyRate Update Finished.', null, 'payvast.log');
    }
}

public function getMerchandiseRemainForAllStockRooms()
{
    if (Mage::getStoreConfig('payvast/general/enable_stock'))
    {
        Mage::log('Payvast Stock Update Start...', null, 'payvast.log');

        $client = new SoapClient(Mage::getStoreConfig('payvast/general/url'));
        $products = Mage::getModel('catalog/product')->getCollection();
        $products->addAttributeToSelect('sku');
        foreach($products as $product)
        {
            Mage::log('Updating Stock for '.$product->getSku(), null, 'payvast.log');
            $date = date("Y-m-d");
            $fiscalPeriod = Mage::getStoreConfig('payvast/general/fiscal_period');
                    $username = Mage::getStoreConfig('payvast/general/username');
                    $password = Mage::getStoreConfig('payvast/general/password');
            $params = array(
                'merchandiseCode'   =>  $product->getSku(),
                        'date'                  =>  $date,
                        'fiscalPeriod'      =>  $fiscalPeriod,
                        'username'      =>  $username,
                        'password'      =>  $password
            );
            $response   =   $client->GetMerchandiseRemainForAllStockRooms($params);
            //Mage::log($response, null, 'payvast.log');
            $stockItem = Mage::getModel('cataloginventory/stock_item')->loadByProduct($product);
            if ($stockItem->getId() > 0 and $stockItem->getManageStock())
            {
                $qty = $response->GetMerchandiseRemainForAllStockRoomsResult;
                if ($stockItem->getQty() != $qty)
                {
                    $stockItem->setQty($qty);
                    $stockItem->setIsInStock((int)($qty > 0));
                    $stockItem->save();
                }
            }
        }

        Mage::log('Payvast Stock Update Finished.', null, 'payvast.log');
    }
}