You'll have to edit your english.php file.
The core file is in:-
includes/languages/english.php
Open a COPY of this file on your hard drive, with a proper TEXT EDITOR such as NotepadPlus, or Crimson Editor. Do NOT use editors that format text.
The phrase occurs in two places...
On or near Line #513 : define('TEXT_LOGIN_FOR_PRICE_PRICE','Price Unavailable');
On or near Line #520 : define('TEXT_AUTHORIZATION_PENDING_PRICE', 'Price Unavailable');
Change the text between the two latter single quote marks, making sure to leave the quote marks intact.
On or near Line #513 : define('TEXT_LOGIN_FOR_PRICE_PRICE','YOUR TEXT HERE');
On or near Line #520 : define('TEXT_AUTHORIZATION_PENDING_PRICE', 'YOUR TEXT HERE');
SAVE the file, then you must put it into your CUSTOM template folder. (On a standard install, this is the " classic " folder).
includes/languages/classic/english.php
(If you use another template)...
includes/languages/YOUR_TEMPLATE/english.php