Ask HN: Inexpensive robot to teach assembly programming?

I am teaching a course in Computer Architecture using NAND2Tetris; however, the book does not cover interrupts. When I was an undergrad, we used robots with a Motorola MC68HC11, which had a very simple instruction set. We wrote a maze-solving program, read data from the sensors, and operated the motors ... all in assembly.

My department has a (tiny) budget to buy robots for the students, but the ones I've found are $100 -200, and they are mostly intended to be programmed using a higher-level language (ARM microcontrollers).

Does anyone know of an inexpensive robot that can be programmed using assembly practically?