This is good advice that has also stood me well over the years.
If you are doing custom development you can also use this at a finer level of granularity: charge a lot for parts of the project you’d rather not do (even possibly pay a bonus for the poor person on the team who draws the short straw and has to do that part, and maintain it) and charge less for the parts the customer might be on the fence about but you know will either make them happy (though they don’t realize it yet) and/or help you get future business elsewhere.
To expand a little: look over the horizon. Imagine you are thinking of expanding into the banking market at some point down the road.
Customer A says, “hey, would you add a feature to your emoji generator to interface with the ACH system?” Stupid request, maintenance nightmare. But you might get paid to have one person get some exposure to the horror show that is banking APIs. Exposure that might later help you think a bit better when it’s time to start thinking about that market. Of course charge a LOT because it’s new and charge a LOT for the maintenance contract. Maybe it helps you estimate engineering and marketing budget, or maybe it helps you decide on a different expansion path.
The converse is true too. Customer B wants hospital integration so they can add cute emojis to messages like “hey, you’ve only got four months to live. Have a nice day!” You’re not stupid so you know you never want to get anywhere near the tarpit of medical software. So you can confidently say “no” (phrasing it, “we don’t believe we could honestly provide the necessary level of support you’d require and don’t want to let you down blah blah blah”).
If you are doing custom development you can also use this at a finer level of granularity: charge a lot for parts of the project you’d rather not do (even possibly pay a bonus for the poor person on the team who draws the short straw and has to do that part, and maintain it) and charge less for the parts the customer might be on the fence about but you know will either make them happy (though they don’t realize it yet) and/or help you get future business elsewhere.