Personally, I'm doing it mainly because everyone else is (stop laughing, it's an important heuristic that should only be overridden when you have a definite reason).

But it does make some sense, I think. A talk isn't a fungible widget. Different levels of inconvenience to talk at different places and different times. A website that won't tell me how much a fungible widget costs - now *that's* annoying.

In general terms, I'd say that my price is in the $$ range, as opposed to $ or $$$$.