You can reuse the medium until it starts to change color. You do NOT want to be paying all that money for new medium for only a few uses.
I use walnut shell through Cabela's (cheaper than Amazon, usually). Fill it perhaps 65 percent of the way, dribble on some FLITZ polish. Just a few "lines" like you are putting chocoloate syrup on top of ice cream. I usually tumble for at least 24 hours.
Best to degrease and clean before tumbling to preserve medium and improve speed toward polish.