It's typically based on the number of friends the Facebook user who Liked you has. This is the formula:
Example: A user has 356 Facebook friends and clicks Like on one of your URLs. You will lose 8 credits.
We also integrate with 3rd party services that provide Facebook Likes. When we are unable to determine the # of friends a user has, you'll be charged a fair number of credits (based on the average friend count system-wide).