Schools are not designed to calculate the actual cost on a per student basis.
Big ticket items like a dedicated SPED department, or a professional working 1:1 with a student can be accounted for. But if a special needs child participates in a standard class (which they do) and the standard teacher needs to do more than average work to accommodate them; that cost is not earmarked for that specific student. Once the bean counters see it, it is just "teacher salary", which gets averaged out across all the students.
Big ticket items like a dedicated SPED department, or a professional working 1:1 with a student can be accounted for. But if a special needs child participates in a standard class (which they do) and the standard teacher needs to do more than average work to accommodate them; that cost is not earmarked for that specific student. Once the bean counters see it, it is just "teacher salary", which gets averaged out across all the students.