IF you could rent something like a sony snap lab and print onsite, that would be your best bet. Definitely have an assitant and laptop there to sell immediately.

As far as pricing, I would do packages. Mainly 4x6 and 5x7 some with 8x10's also.

A santa shoot is more about quantity vs large sales. I'd start my packages at the $25-$30 mark and go up from there. Honestly, I really don't know as I don't do them, but, I do remember buying them! People will buy and I remember spending about $30 on a couple of photos of my kids in the mall