You're about to make a fortune "off" us?
SCRIPT
Bree smiles and introduces herself and Rex to Dr. Goldfine. (A marriage counselor)

Bree: "And I brought you some homemade potpourri." She hands it to him and walks inside.
Rex is left behind with the doctor. "The answer is yes. You're about to make a fortune off us."

QUESTION
I don't get what "off" means in this sentence.
(You're about to make a fortune off us.)
Fortune for us? Fortune from us?
What do you think? Thank you in advance!

Question

You're about to make a fortune "off" us?
SCRIPT
Bree smiles and introduces herself and Rex to Dr. Goldfine. (A marriage counselor)

Bree: "And I brought you some homemade potpourri." She hands it to him and walks inside. 
Rex is left behind with the doctor. "The answer is yes. You're about to make a fortune off us."

QUESTION
I don't get what "off" means in this sentence. 
(You're about to make a fortune off us.) 
Fortune for us? Fortune from us? 
What do you think? Thank you in advance!

Answer

You will make a fortune off us = You will make a great deal of money because of us.
In other words, they will be paying for a large number of visits to the marriage counselor because their marriage has a great number of problems.

Answer

"Making a fortune off us" means "Bree" is getting a lot of money from "Rex".

Unfortunately I cannot make a diagram easily on a computer, but I will try.

EDIT: sorry diagram didn't work out, so I will clear it