Hi
You won't have "pure" magnesium castings, because magnesium by itself can be quite a reactive element. Have a look on youtube for videos about what happens when you put magnesium into water...
The castings will be of a magnesium alloy, of which there are lots and lots of different types. Your best bit would be to get a sample, take it to a yard and get them to hit it with their XRF gun so they can grade it and give you a price.
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