About ten or twenty bucks.
Investigations in Florida revealed how much food service workers earned for selling payment card numbers to crime networks.
When you hand your card over to a waiter (or use it at the drive-through window), they have a chance to "skim" data off the card. The most efficient way for them to do this is to swipe the card through a small device that stores the information for easy downloading later.
Most employees do not do this, but you should think twice before handing your card to somebody who can take it out of sight. It only takes a split second to swipe the card, and really talented scammers can do it in their pocket without you noticing.
This should also make you think twice about which cards you use - is it risky to use the debit card from your primary checking account every day? Assuming you don't pay interest, would it be better to use a credit card so you don't risk overdraft charges? Consider the tradoffs of using debit vs credit cards in everyday life.