MAC's Mixing Medium retails for $14.50 and is used with pigments. I used this to use my pigments as eye liner and it stayed put really well. I noticed that the pigment shades looked more vibrant using the Mixing Medium. I also used this with some of my eyeshadows to use as a liner and it worked great for that. Another cheaper alternative to this is Ben Nye's Liquiset which works just as well.
This is interesting stuff...I was originally using benefits shelaq for pigments to make them into liner but that was drying up too fast. This stuff worked pretty well but it doesn't work with matte pigments. It takes some time to master how much to use with how much pigment but its really inexpensive ($14.50) and the bottle is pretty big.