I live in South Africa, but I assume wheelweights are the same everywhere. Yours clip on to the wheel rims like ours, don't they ?
I'm a commercial caster, and have measured everything. Several years ago I weighed about a ton of wheelweights before melting. I got, as near as dammit, 75 pounds of alloy from 100 pounds of wheelweights. It was not a single melt (obviously), it was melted in several batches, and the interesting thing was the consistency. Never varied outside two or three pounds over or under 75 per 100.