Hi,
When you edit the file product / listing_price.php via the menu Display>Views, you have this line of code which displays the price with taxes of the product on the product page (and on the listings):
echo $this->currencyHelper->format(@$price->price_value_with_tax, $price->price_currency_id);
So supposing that you want that change on both listings and the product page for all the products, you could change that line to:
echo 'Purchase Price '.$this->currencyHelper->format(@$price->price_value_with_tax, $price->price_currency_id). ' or '.$this->currencyHelper->format(@$price->price_value_with_tax/10, $price->price_currency_id).' per month of 12 months';