If someone wants to pay you $100, you simply have to explain to them that the request is unreasonable – then, walk away. Tell them to call you when they have a budget. And remember, maybe they just honestly don’t know. (Never try to do business with someone who “doesn’t know.”)
Very early on, I glommed on a book about consulting contracts by the late Herman Holtz (nee: Hermann Holz), who wrote several good ones. He described a system of task-orders and change-orders, and binding estimates.
And then there’s Judge Wopner from the old People’s Court television program (which he pioneered):
"Get it in writing!"
A simple letter, signed and dated in pen by both of you, with an autograph copy held by both of you, not only constitutes a legally binding agreement but also compels both of you to sit down in good faith and to define and negotiate the project, or whatever it is that the letter contains.
“Good fences make good neighbors.”
If you make a verbal agreement, get it in writing. Even an e-mail will help: _“Okay, Bob, yesterday, January 10th 2019, you called me up and we had the following discussion. Here’s what I think we said and agreed to. Please review it and send any corrections – or, if you agree, please confirm it. Thanks.” Then print a paper copy of that e-mail and the response and put it in your paper files. "Diligence matters."
This line of work is very labor-intensive and that makes it expensive. Changes, also, are expensive, and “what makes it expensive” might never be visible. But, professionals all around the planet are likewise … “expensive, but worth it.” For the good ones, their reputation precedes them.