price guide for used ipads

hi, i don't know if this is against the rules but I've been in a market for a used ipad for school.
im interested in buying either
air 4th or 5th gen
mini 6th gen
or any 11" pro (but those might be out of my budget)

there aren't many used ipads for sale in my region so i don't know which price is reasonable to buy at.