This is voluntary though. Sure the odds of more volunteering increases with the total number of bodies, but I don't think it'd be actually significant.
Sure. There isn't an inherent financial incentive here for the prison system (though an illegal one could easily develop, as it does with all else). It really boils down to the type of study and how much time gets exchanged: I'd assume the more dangerous something is, the more "time served" it's worth, for example. And of course there are all the usual caveats of determining a fair rate, ensuring informed consent, etc.
