Hello Dr. Scarfe,
Would £0 fit into your budget 
Your VIEWPixx 3D display has this functionality built-in. Using “Pixel Mode”, you can drive digital outputs based on the top-left pixel of the display, with microsecond sync to video. This single pixel is much smaller than a photodiode patch; and RGB bits can drive multiple digital outputs, for things like encoding a trial ID. Sort of a super-photodiode.
Regards,
-Peter April