for portraits session charging a sitting fee method, usually only giving customer images proof to select photos to be enlarge and print, the customers does not have all the images after the job closed. so the sitting fee is not that high, but the prints are charging a premium.
most local portraits studio is adopting this method.
if you want to give away all images to customers, you have to price your service high enough, means you have already make your profit when you hand the CD to your customers. don't expect they will come back to you for making print enlargement.