Before you go on a trip you have to add up the cost. Costs for a location shoot when you DON'T have a storefront studio would include the gas for your car, the cost of your time from the time you leave home to the time you get back, plus the cost of your time dealing with post-production, and a sum that covers the cost of operating your car and equipment for the time you are using them.
Add to this the cost(s) of producing the materials of the sitting, such as batteries, film and processing if you're not digital, reprints if you've sold them as part of the package, and any other accessories, INCLUDING YOUR TIME TO ACQUIRE THEM. If you spend an hour visiting shops to get the picture frame you sold then it HAS to be paid for, just as if you would have been paid by an employer.
Lastly, on top of it all there MUST be a profit margin, even after you've "paid yourself by the hour" to do the shoot. This is because Uncle Sam (and maybe your state and local municipality as well) want's their cut of the action to help pay for your use of the streets and parking lots and police protection, etc. that you used to make the photographs, as well as to cover any error you may have made in calculating all of the above.
Okay, you've added it all up and it comes to $X. This is what you MUST make from the sitting, and it doesn't matter WHO you are photographing, you WON'T shoot the sitting for less. It helps if you have a PRINTED pricelist beforehand, spelling out all the charges IN WRITING. You'd be surprised at how much this cuts down on arguments over the price. You also will have a SIGNED AGREEMENT ahead of time with your client that gives you the ownership rights to the images, and that they can be used for self-promotion at any time in the future WITH OR WITHOUT notification.
I used to sweeten the prices by having my minimum necessary price available for those who paid in full in advance advertised as a TWENTY PERCENT DISCOUNTED PRICE. Any other payment method, other than for additional reprints, was FULL price, which was actually 25% higher than the discount, because 20% off a 125% price equals 100% of the discounted price. Check it with your calculator if you don't believe me. Say the minimum price point is $100. Take 20% off of $125, then take 20% off of $120. Which comes out right?
Down the road, when you've built up your practice, you can work from the other end: you already know how many sittings you do per week, and you know how much it costs you to be in business AND cover your personal expenses such as food, clothing, and shelter, AND still have a little to put in your retirement and savings accounts, then total costs and profit margin divided by number of sittings equals how much you MUST charge per sitting. Charge less and you die. That's all there is to it.
31 years of professional photography.