This lines here echo $this->currencyDisplay->createPriceDiv ('basePriceVariant', '', $prow->prices, FALSE, FALSE, 1.0, false, true);
or the line you need to change
echo "<span class='priceColor2'>" . $this->currencyDisplay->createPriceDiv ('billTaxAmount', '', $this->cart->cartPrices['billTaxAmount'], FALSE)
When you go to the function createPriceDiv, you will see this
//The fallback, when this price is not configured
if(empty($this->_priceConfig[$name])){
$this->_priceConfig[$name] = $this->_priceConfig['salesPrice'];
}
So you need to provide the $this->_priceConfig['billTaxAmount'] with the right array $this->_priceConfig['billTaxAmount'][0]=true and $this->_priceConfig['billTaxAmount'][1]=2 and $this->_priceConfig['billTaxAmount'][2] = true.