Hi Lawrence
This is a question that I have never been asked before. I Dont know the answer to this though I would imgine it could lift a fair amount of weight.
I would suggest that the easiest way to find this out is to add weight to the rappy steadily till it is able to fly safely with the weight on it. As long as you weigh the item then strap it firmly between the skids and as close to the centre of gravity as possible (This is normally directly under the main rotor shaft) then be sensible. I really dont think it will take long for you to find out.
Please be aware that when you do have weight under youre heli the inputs need to be steady not fast or hard. The heli can quickly get out of shape, Even more so if you do not ballance it properly. You also need to make sure that the weight added cannot slip out of position (It Must Be Firmly Held)
Good luck
Let us know how you get on.
Darren