Cost Effective Laser Detection
you could assemble a photo-resistor array, if you're looking for a cheap, small, precise measurement of the location of a dot. You will need a number of Input equal to the number of rows + the number of columns, wired up like so:


 instead of LEDs, use photoresistors. wire all 'R' wires to a 5k pullup, and all 'C' Wires to a 5k pulldown 

You can buy a hundred of these for about 10 cents a piece, probably less, and make a 10x10 matrix for 10 USD.

if you don't need that sort of precision, I agree with ambitoise, and check out some image recognition solutions.