How do I override a function from a module?
It's Drupal...there's always a way, but the amount of time it will take to do might make you think twice :)

If you look a bit further up the food chain so-to-speak you'll see that this function is exclusively used by commerce_price_table_field_formatter_view(), which declares a field formatter that is used for the commerce_price_table field type.

With that in mind, you can quite easily implement your own field formatter, assign it to the commerce_price_table field type, and use as much custom code as you want, all the time keeping in line with best practices.

Basically you need to implement hook_field_formatter_info():

function MYMODULE_field_formatter_info() {
  return array(
    'MYMODULE_commerce_multiprice_default' =&gt; array(
      'label' =&gt; t('MyModule Price chart'),
      'field types' =&gt; array('commerce_price_table'),
      'settings' =&gt; array(
        'calculation' =&gt; FALSE,
        'price_label' =&gt; t('Price'),
        'quantity_label' =&gt; t('Quantity'),
        'table_orientation' =&gt; t('Orientation'),
      ),
    ),
  );
}


And then implement hook_field_formatter_view(), field_formatter_settings_form() and (optionally) hook_field_formatter_summary(). 

For each of these functions just take the code from the same function in the contrib module, and make the changes where you need to.