My favorite cheap Casio is fx-115ES Plus 2nd Edition, $17

https://www.amazon.com/Casio-fx-115ESPLS2-Advanced-Scientifi...

Includes GCD and LCM, some of the newer ones don't have them.

If you want graphing, there is the newish fx-CG100 has a nice display, but they removed Casio basic, it now only has micro Python (way too awkward to type on a tiny keypad):

https://www.amazon.com/Casio-ClassWiz%C2%AE-Calculator-Funct...

The older ones that still have basic:

https://www.amazon.com/Casio-fx-9750GIII-Graphing-Calculator...

BTW, here is a review I made of many calculators, measuring keyboard efficiency: (HP-15c still the best)

https://github.com/jhallen/calculator/wiki