DISCLAIMER: I haven't heard of this kind of trade deal at IWM, but I have traded pewter melt like-for-like for new miniatures with another company. A human-sized 30mm miniature tends to run around 1/2 an ounce. More for a chunkier mini.
I haven't weighed one, but, at a guess, a battlemech mini would probably be 2-4 times that weight, depending on the sculpt.
So.... at least an ounce, bur probably no more than 3 ounces. Does that help? Obviously, something giant, like the Tonbo, or Poseidon would weight significantly more.