Adoption is expensive because the process to legally adopt a baby requires the involvement of attorneys, social workers, physicians, government administrators, adoption specialists, counselors and more. While the adoption journey is an emotional one for prospective birth mothers and adoptive families, the adoption process is a legal function. Adoptions completed by fully licensed agencies are held to high ethical standards, which can mean more paperwork and higher costs.
Usually, a shelter has already attended to the spay/neuter, the initial shots, and a veterinary clearance exam. If the cat's original situation left it with any medical challenges due to poor nutrition or neglect, these have usually already begun to be addressed by the shelter. You may get to play a final role in nursing your rescue back to full health, creating an initial bond between you both.