Actually, you can make your own water-based mixing medium if you wanna spend the money on something else. It works just as well...you can try it before you go to MAC Pro, it doesn't cost much to make, so if you don't like it, then you can still get the MAC one, but like I said, VERY similar. The recipe:
1 Part Glycerin
3 Parts Water
And shake em up! Glycerin can be found in the first aid aisle, my bottle was only $3 CAN and I barely used any, I'll be makin Mixing Medium for a while.