I have no idea, but any particular reason for wanting to substitute?
Unless you have a some on hand, I doubt that it would be cheaper than the dolo. I pay less than $5 for a 40lb bag at Lowes.
Dolomite is not the end all for pH and cal/mag. There are other things that work well, sweet lime, oyster shell flour to name a couple. Mostly it comes down to what you can source the cheapest.
Wet