I think I have run across some code that involved calculating when you get close to the stall rate of a motor. Don't ask me how to do it, but I have seen it somewhere in these forums, and it seems that it wouldn't involve any further purchases.
For now I'm going with a small electromagnet as end effector.
Could you explain your end-effector idea a little more?
Paper clips, wow, I would think Lego blocks might be a lot easier. Or even eggs.