magento 1.9 – Add custom text / price for product editing in admin (not custom attribute or editable field)

I have a business that uses prices without taxes.
I would like to add the price. Tax under the price field as text (not as editable field or attribute, only as information).

I've already added a new column to the product grid showing the price including VAT (with a renderer that returns $ this-> helper (# tax; #) -> getPrice ($ product, $ product-> getFinalPrice (), true);)

But I just can not figure out how to edit this on the Edit Price tab for the product. If I had to add an attribute, it could, in my opinion, be added to Attributes.php (Mage_Adminhtml_Block_Catalog_Product_Edit_Tab_Attributes), but can I add an info block / comment / text line?

It does not need to be calculated in real time the price when you enter a price in the price field, only when saving / loading the product (with PHP)?

In summary, I add a custom comment / info text under the price field.

I want to show $ this-> helper (# tax; #) -> getPrice ($ product, $ product-> getFinalPrice (), true); in this comment / info text.