What is the adverage cost just to change the name of an executor of the will?

My parents had a will drawn up years ago. My father was taken to the lawyer last year to sign POA. Because he has very poor hearing, he was unaware that my sister had a new will drawn up and she made herself executor. My parents would never have done this. It appears to be an example of "undue influence". My dad needs to change the executor but I have to pay for it because she has all his money. I'm on Disability and need an idea of cost so I am aware of what to expect.