I've commented on this quite a bit. Special education services, hospitalization, counseling services, social workers, security, resource officers, physical and occupational therapists, speech language pathologists, audiologists, aids for students who suffer from seizures, severe autism or cognitive disabilities, blind students, etc. Let's then throw in the cost of interpreters, for each of the languages spoken in CPS, which is dozens (not just Spanish). All of those aren't cheap, especially interpreters, they aren't giving discounts to schools, they are extremely expensive, especially for the very rare languages.
Special education is a huge cost. Clark High School, a CPS school, which is highly respected and considered by many to be the second best under Walnut in CPS, has 13 Special Ed teachers for approximately 700 students grades 7-12. No private schools have a special education population like that.