I have had customers demand VB.NET over C# because "C# developers are more expensive." I don't care whether my guys type semi colons at the end of the line or not and I don't adjust their pay based on it.
You know that guy who says "I've been rich and I've been poor, and rich is better"? Well I've had customers who tell me what language to write their code in, and customers who don't, and with the exception of writing demos and sample apps (a line of work that has tremendous benefits), not telling me how to do my job is better. Some of my best customers remain focused on their business problem and trust me to solve it. They don't know the difference between Java and PL/1, never mind Java and Python, and they don't care either. They have stuff to ship of their own.
If you don't have the option of not having these people as a customer, don't try too hard to get them to change their tech. They have their reasons and they might even be valid (knowledge of the maintenance team, knowledge of the IT team who is going to run backups or configure servers or open ports, license costs, security risks, availability of alternate vendors.) About the only thing that does work is "I can't do it in the tech you're asking for (at any price) but in the tech I like I can give you more than you're asking for, for about half your budgeted amount." I've done that once and succeeded.