We did the $45 package (hair and nails) and I feel this was enough to pay! They really enjoyed it, and already had dress up clothes anyway. We did look in the shop and offered to buy each daughter a princess dress, but they were all to itchy for them, and they both decided to get an Alice dress instead.
I would say, if your daughter already has a princess dress, wear that, and just get the hair and nails. When they are finished they get to keep the little crown in the hair, and also the little pack of makeup and nail polish, also they get a pink sash to keep.
If you are going to do it, do it as early in the day as possible, and maybe try to get lunch with the princesses somewhere.
Although we only had the medium package, not the expensive one, we still had the photo shoot, done a few buildings away. They put the photos on to our photopass, so we had the option to view and buy them when we got home.