Question about counting game props in physics sim

I have a project I am working that has come across a stumbling block. In the scene, I can add 3 different objects, each with a different integer prop 1, 5, 10. I add object them randomly into the scene where they get collected on a tray. All of this happens inside of the physics simulator, so some of them roll off the tray, which is how it is supposed to be. My goal is to be able to count the total of the integer props of the objects on the tray, but I can’t think of a way to do it with logic bricks or my limited knowledge of Python.

My first question is this: Is it possible to count the integer props of objects in an area at any specific time?

I can’t count the integers as they are added, because some fall off the tray. Like wise, I can’t count them as the collide with the tray, because they can fall off. The closest way I can think of currently, is to add them as they come out and put in an invisible senor object that they collide with when they fall off the tray, subtracting from the total.