Laser_Developer:
OK, your price range looks reasonable.
It's possible to make a 2D position sensor based on one or two scanning lasers. Normally, these are very expensive devices, but keith1024 has been modifying SF02/F lasers to do area mapping Laser Measurement (SF02) Setup & Library - Sensors - Arduino Forum
It may be possible to place these in the back corners of the stage to provide a 2D map of anything on the stage area. I think that keith1024 is going to provide mapping software for the Arduino so I'm sure you could ask him to modify it for your application.
Here is the reply from keith1024:
I have read through your thread though and I think I have a good picture in my head of what you'd like to achieve. I can see how you'd be interested in the SF02/F wide beam angle, and I can tell you that it is actually very directional. However, for what you are trying to do, I believe it is completely doable with two scanners (SF02/F) mounted in the back corners of the stage with their zero points at -45 and 45 degrees, they would then do a sweeping scan, driven by a servo, that covers 0 to 90 degrees (so the entire stage), and I think this would give a good symmetrical view of the area.
Experiments with the SF02/F have shown that we are able to pick up an ~2cm wide object, in a 90 degree scan done over ~1 second. I think if the target is something as wide as a person, it'd be possible to perform the 90 degree scan in even less than 1 second.
It is a fantastic idea!