Given that we're not great at communicating with animals, what level of reward would be considered justifiable for participating in the study?

As in they could get a reward for starting the study...

Also, isn't this sort of "voluntary" testing a little unethical in itself? For example, testing an addictive drug on a rat, they don't know the downstream consequences since there is limited communication, but the immediate effects might be incredibly gratifying. It would lead to high "volunteer" rates but still expose them to massive harm.