Look closely at a selection of wheel weights. You will notice that the size of the wheel weights varies. However, the size of the steel clip on the wheel weight does not. Therefore, there is no way to accurately estimate how much of a selection of weights is going to yield. Nevertheless, you could take a representative sample, weigh it, smelt down the weights and weigh the lead recovered and do the math. If the rest of your supply of WW's have the same general distribution of large, medium, and small weights, you should be able to figure it out.
However, worrying about such trivial matters is the past time of anal-retentives.