A general answer and what I have done with some kids that have extreme anxiety is to just do happy visits until I earn the trust of the kid. We would just do small things. Like ride in the chair and play with my air water and the suction. Baby steps and many happy visit until she had a relationship with me and like I said trusts me and had only fun appointments.
This has worked and if takes time and the insurance doesn’t pay for happy visits but they don’t cost much. It’s worth it in the long term as she will learn it’s not a scary place. One kid we did almost 15 of these visits until he felt comfortable and had no anxiety for cleanings. Building this positive relationship now is important for her life long view of dentist. That’s my advice.
I can not speak for all dentists but where I am from many do offer payment plans.
Some like myself offer the plan interest free. Some refer to a third party that offers the payment plan. This type of third party payment usually comes with very high interest rates and possibly administrative fees. You should be able to call around and find a dentist that doesn't practise this way and just spread the payments out without charging you extra. Again I have no idea where you live or which country but in many parts of Canada where I live that is how it is often done.