How would you go about determining a price for this service?
Pricing is not only locale dependent but also target market dependent within that locale, among other things. For example, a city like San Diego supports a Chevrolet dealer, a Tesla dealer, an Audi dealer, and a Lotus dealer. There's also a healthy used-car market. The prices for all of these things are going to be different here than in, say, a small town in the midwest.
So, who are you in this analogy and who is your market? Without knowing more about the area and clientele and about the exact service being asked, it's impossible to give any meaningful advice on pricing.
I've linked below some questions on pricing for weddings - this can easily be adapted to your daughter's situation.
My daughter is 16...
My own first professional portraits experience also took place in high school - shooting portraits of a teacher's kids at the park. I didn't charge for time and charged nominally for prints (something like $7.50/sheet). At the time, I had very little portfolio and was trying to build myself up. I fell into the trap of shooting things for "experience." Do not fall into this trap! What little portfolio you have is obviously good enough to get you the gig - your skills are valuable.
Most amateurs going pro in any business feel guilty for charging for their skills. I would say that this is natural - how can I charge so much for something that comes so easily to me? If your daughter feels this, then figure out what the ideal price point is for her and put a plan together to get there over her first few gigs.
If your daughter has not yet held a job or participated in any other business activity - then now is the time to begin learning these things. The initial focus should be on operating a business - not on making profit. It really doesn't matter what she charges, as long as it's acceptable by the client and she doesn't lose money on the deal. Break-even is A-okay.
Instead of focusing on pricing, help her to understand her photography as a business. List her gear as assets with a depreciation schedule, list operating expenses, look at future gear as investments that need to be supported from the business coffers, list what she makes as income and deduct costs to show how profit comes to be. If she really wants to go it as a pro, she needs to develop keen business, sales, and marketing skills. Many a great shooter has washed out for lack of these skills. Many mediocre shooters are in business because they have them.
After this first job and going through a business building exercise, you should look at competitors in the market to see what they charge. Have her plan for being self-sufficient, paying for her rent, car, food, insurance, etc. from her photography money alone. What sort of income do you need to survive?
All of this will influence pricing. When you've come up with what you'd need to charge per session and the minimum sessions per month - determine if that is realistic within your market.
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