Hmm. Bronze is 90% or more copper, and 10% or less tin or arsenic. It is an easy alloy to make.
So your bp contains about 0.9 cp worth of copper, and 9.1 cp worth of tin. If 1/10 of a coin weight of tin is worth 9.1 cp, tin is worth 91 cp per coin weight, or nearly as much as gold!
No wonder...