If you can let go of the backlight requirement, then the TI-89 is great value used. They go 15-30€ range. They were TI's flagship calculator at some point. Well

If you can let go of the backlight requirement, then the TI-89 is great value used. They go 15-30€ range. They were TI's flagship calculator at some point. Well-known, and extensively reviewed. Many consider its TI nSPIRE successors to be flawed. Avoid TI-83/84, despite fancy-looking backlit screen and smartphone-inspired UX, they are ultimately the successors of TI-82, the "high schooler" stripped down functionality version of the TI-85.