This is silly, but...
My son's school supplies cost less than $20. He is very good at conserving school supplies and holding onto them. He has only gone through 7 glue sticks in 3 years. We bought a 10-pack before kindergarten. Every time I go into an elementary classroom, I wind up picking up broken crayons from the floor and regular kids go through a lot of glue sticks.
His back to school clothes were 2 pairs of sneakers for less than $20. Mostly this is because he hasn't grown. All his old clothes are in good condition and still fit. He wore out his favorite sneakers, so these are replacements.
I know we won't be able to get away with under $40 in the future, especially once he starts wanting to wear brand name clothing.