Low-cost long-range movement detection
Maybe you can rig a standard motion-activated light to work for you: add a lens or pipe in front of it so it looks only at the point you are interested in? 

They work with a PIR sensor and a Fresnel lens, which focusses alternate parts of the field-of-interest on and off the sensor, so a movement from an 'on' part to and 'off' part or vice versa gives a large chance.