Which item provided for projects with Mathematics as a primary subject is the most expensive?
Mathematics as a primary subject refers to primary_focus_subject = 'Mathematics'; most expensive refers to max(item_unit_price)
SELECT T1.item_name FROM resources AS T1 INNER JOIN projects AS T2 ON T1.projectid = T2.projectid WHERE T2.primary_focus_subject = 'Mathematics' ORDER BY T1.item_unit_price DESC LIMIT 1