they exactly weigh as much as they are worth.......... nothing
Lol Jokes. Here's the real answer;
As shown by this youtube video and another Question on Wiki Answers the weight for a regular SINGLE Pokemon Card is about 1.7-1.8 grams. It can easily vary by .1 or .2 grams
However, the Youtube Video also shoed that for Holofoil Cards, the weight was normally around 1.9-2 grams
For the record 1 gram = 0.00220462 pounds.