No one's exaggerating the amount of time and effort that goes into sessions, but if it really did work out like that you'd see a lot more experienced dommes charging less. Plenty of us have experimented with lower rates, following pretty much this exact logic. The results are not what you'd expect.

The truth is that a lot of clients will compare those two Mistresses and decide that Mistress A must be better if she's charging so much more and can get clients to pay it.

Also, unfortunately: a lot of the bargain-hunters are absolutely fixated on getting the most for the least, making them demanding, unpleasant, entitled clients. This is not to say that clients like this do not exist at higher price points -- but at least there they're paying enough to possibly be tolerated. Nor is this to say that lovely clients never see cheaper dommes, because they definitely do! But the bullshit-to-cash ratio does seem to get much worse as rates go down.