Inexpensive method for highlighting a sketch
I wonder what you mean by highlighting a sketch? If you work on a white (or almost white)  paper then the only thing you gonna need is a soft (6b) pencil or even cheaper a regular HB penci to start. But I recommend a 6B.
If you gonna work on a toned paper or cardboard then be aware that chalk won't work well on graphite. Also a black and a white one will give you a slight blueish cast if mixed. Thereby going with a 6b and leaving the highlights untouched is better. If needed you can use an eraser as a cheating option.