Free is More Expensive
When a person who normally charges $100 for their services gives you a pro-bono session, what you are getting is a service that is more expensive than their going rate.
It’s not because they are losing out on $100. Money can be supplemented.
What they are losing is time. Time they could spend on their work. On higher paying clients. Time having sex. Or sleeping. Or eating a really delicious meal somewhere.
Anything else but doing something for you, for nothing.
If you cannot see the real cost of someone doing a favor for you, it’s probably cheaper to pay them the $100.00.
(On the flip side, if you are constantly giving away services you normally charge for, you might want to reconsider what you value.)