240, not real definite about this but I have heard that similar growth studies are being conducted in a few other US cities, but that is the first time I've heard anything about the financial remuneration.
I would assume it would be much higher if it involved a substantial period of time and if there was any physical risks involved.
At one time there was a medical website that searched for volunteers for similar studies some of which were more that likely interested in growth research / muscle enhancement.