Text option in OpenCart product, how to add to the cost?

Question

PHP

Hello, I have such a problem. There is a site on Opencart 1.5.5.1 in which the products have a text option "calculator". How to make it so that when adding a product with this option to the cart, the filled value is added to the cost of the product. I found what I need to dig into system\library\cart.php
line 137:

} elseif ($option_query->row['type'] == 'text' || $option_query->row['type'] == 'textarea' || $option_query->row['type'] == 'file' || $option_query->row['type'] == 'date' || $option_query->row['type'] == 'datetime' || $option_query->row['type'] == 'time') {
$option_data[] = array(
'product_option_id'       => $product_option_id,
'product_option_value_id' => '',
'option_id'               => $option_query->row['option_id'],
'option_value_id'         => '',
'name'                    => $option_query->row['name'],
'option_value'            => $option_value,
'type'                    => $option_query->row['type'],
'quantity'                => '',
'subtract'                => '',
'price'                   => '',
'price_prefix'            => '',
'points'                  => '',
'points_prefix'           => '', 
'weight'                  => '',
'weight_prefix'           => ''
);

I tried to add any number to the price value in the array here and put "plus" in the prefix, but it is not added to the cost of the goods in the basket.