I charge $5 for 4-8 week olds, $8 for started pullets and $10 if they are closed to laying. If I get a breed that's harder to come by, I will charge more depending on the quality.
Concidering that most places charge $2-$3 for a chick, and the cost of feed and the time invested, $7 seems reasonable for a chicken that's laying. The big question is how long has she been laying, if she is 3-4 years old, I would not pay as much as she may not be laying for much longer.
edited to add: see if you can work out a deal with your dad to work off the difference