This question depends on so many factors, but ultimately, it's really up to you. Since you are just starting out, the easiest thing to do would be to estimate how much time you think you'll spend preparing for the session, creating the images, editing, and preparing for delivery. Then decide what hourly rate you'd be willing to do all of this for and multiply that by how many hours it will take you.
For example, if you think you will spend a total of 8 hours and you are willing to work for $10/hr, then charge around $80. If your time is more valuable than this, then charge more. If you are counting on print sales on the back end, then you can back off the session fee and plan to get paid from print orders.
The only way to really know is to put a price out there and see if it sells.
Once you become more established and actually have commissioned clients, then I would strongly recommend evaluating more closely your costs associated with doing business and making sure that you are being properly compensated. Many businesses go under because they try to undercut the competition with low prices, only to realize later that they aren't making enough to sustain a profit.