When my clients ask me for 5x7 I know that they are not inquiring about size but they are trying to see if they can afford me OR they do not know any other size of the photos.
I take my time to explain to every single one of them that cost of producing (my time, equipment, cost of good, cost of running business, insurance, my expertise etc) 5x7 or 8x10 is the same.
Since I am boutique photographer I take limited amount of clients per year. I spend around 20 hours or even more per session. I do not do quick photoshoots, there is plenty of photographers around who do it just like that. Because I take my time and design all the session and photoshoot from scratch my cost of doing business per client is in hundreds of dollars.
Of course I cannot expect anyone to just trust me and shove money into my hand. With that said I do not charge session fee. My service, labor, time what ever you want to call it is included in price of the products. After the photoshoot my clients come back to my studio to see their images. During this appointment I assist them with choice of images and design of their wall galleries.
On average my clients spend $1300. But I had clients who spend as little as $300. It really depends on what they are looking for. There is no such a things of one size fits all in portrait photography.