I know some people use this but I have done custom metal work on off road vehicles and used sandblasters The media i have used is sand, almond shells, glass and slag. Thing is its all bagged at the same place with the same equipment. This equipment cant really be cleaned because there can not be any moisture in the media or it will clog the sand blaster tips. so what im saying is you never know what may be mixed in the bag and the slag is treated with a silicone to prevent rusting and moisture. Here is a post i read a while back. If you want to go cheap I really would go with either pool filter sand or play sand.
I have learned that what I think is a cheap way to go ends up costing more in the long run. Just my thoughts.
I found this on another website..
"Black beauty abrasives also known as coal slag, black diamond, black blast and boiler slag is an inexpensive media. It is one of the safer forms of abrasive media, containing less than 1% or no silica. Black beauty media also produces little dust, however, may release hazardous air pollutants (HAP) into the surrounding air. Black Beauty abrasives are made from crushed liquid coal slag from utility boilers. The abrasive contains iron (Fe), Aluminum (Al), Magnesium (Mg) and Calcium (Ca). The media comes in sharp angular grains ranging in many sizes including coarse, medium, fine and extra fine grained."
(goes by 'black beauty' also)