How much does it cost to obtain legal guardianship of an elderly parent?

You'll need an attorney and to pay court costs, so it will be substantial. Much will depend on how complicated the case is. If it's very obvious that the parent needs this protection, it may not take long, therefore it will cost less. If there are people who may protest or if the medical evidence isn't clear, it will take longer to prove your case, which will make it more expensive.
Good luck,
Carol