Newbie W/ newbie Mud Flap?

Hi everyone. Just bought my Prius and have to say me and wife are in love with it; Great car. Not as fancy as 2004 Nissan Murano ( the gas hog) that we have, but a heel of alot more fun to drive. Probably be selling the gas hog soon and buying another Prius.
Wanted to go out and buy some accesories this week and just wanted to know before I buy them, do mud flaps take away from your gas mileage? If so how much can i expect to loose.