I have 13 years in the field and very few of the individuals I worked with had guardians. Most had guardianship of themselves. There was actually only 2 out of all those I worked with.
For the most part they received their checks and the staff at the group homes or the family they lived with paid their bills then deposited the rest. There are laws about how much money they can have on hand so they often got to go buy new clothes or something they really wanted. Such as a VCR/DVD player and some new movies.
One of the people who turned 18 and had a guardian had a mom who was nuts. She lived with her half brother and his wife. They filed the guardianship so the mom could not come in and say the girl wanted to live with her mom. This is the mom who would duct tape this girl to the bed when she was 12, 13, 14, etc...and sell her to anyone who would give her money for alcohol and drugs. In this case she needed the protection of her brother and sister in law.
The other girl came from an institution setting. She had a mom but the mom had recently passed away. Her aunt took over her guardianship. This woman was smart, worked a regular job, had managed her own money with assistance from staff, and had lived in a supported living style of home.
She had a long time boyfriend that she was sexually active with and had been on birth control for years. She was fully able to make her own decisions.
The aunt came in and said NO SEX! No birth control! No nothing and she took over the money. Within a few months the girl was pregnant and the money for bills this girl needed was gone. Since the guardian had said no sex we had to stop them from dating or even seeing each other at work. It was horrible. Of course they'd find a way to see each other, they had a lovely little girl who had NO disabilities. The aunt tried to sue the facility but it backfired on her. She was told this lady was obviously better able to take care of herself than the aunt was.
The aunt was a religious zealot. Seriously, if she had just had strong convictions about sex outside of marriage or something I could have understood it. But she would stand and preach to us and this girl for half an hour if not longer about how sex was a sin...God made it, not a sin.
This lady finally got to be her own guardian after she had the baby. Staff testified in her behalf and stated her cognitive age, social age, spatial abilities, and more. The judge visited with her and allowed her to take control of her own business.
So, depending on why you want guardianship of this person and can't allow her to have her own guardianship whether she's living with you or in a group home/shared living/supported living situation I don't know why it would cost that much. It's basically her going to the court house and signing a paper stating she willingly gives you guardianship of herself. All it should cost if the filing fee.