That's the game they play. So, when you quote you simply say 'for a package size 'up to xxx pages'; which then allows you to renegotiate immediately--before you print, if needed. Rarely, they truly don't know, but they usually do know at least an approx. or tight range. They just hope you'll do it because you accepted it. Undisclosed stuff should be negotiated AGAIN--immediately. My line is 'due to undisclosed (whatever), I require an increase of $XX--for a total fee $XXX or please re-assign and advise'. GET AGREEMENT IN WRITING. Never failed yet. Just don't be an idiot over 5 extra pages. I use 25 page increments.
Not only does it cost more in supplies, but the larger the pkg., the more time you're at the table.