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A few numbers:
Total District Enrollment 24,796 (The Wisconsin DPI enrollment number for Madison is 25,395).
Open Enrollment Leavers: 772
Open Enrollment Enterers: 175
Much more on outbound open enrollment here.
Tax & spending authority are largely based on enrollment.
The most recent 2010-2011 budget document indicates total planned spending of $373,157,148, which yields $15049.08 per student.