I sometimes happen to be in a dialogue like this:

X: According to GPL, you can sell copies of your software at any price you want, right?

Me: Right.

X: But your customers can in turn sell copies of your software at any price they want, right?

Me: Right.

X: So, the only way a GPL developer can make money is by charge for development of their software?

Me: Not only for development: also for maintenance, improvements, customisation, installation, training, gadgets, support...