Sometimes I buy JoAnn Fabrics muslin to dye, mainly because it's cheap and they have amazing coupons. But if you look at the difference below, you have to ask if cheap is really worth the price.
The mercerized fabric on the left has taken the same dye recipe so much more vibrantly and with so much more texture.
The mercerization process is pretty brutal for the poor fabric. It is given a bath in caustic soda, which causes the fibers to swell, increasing the surface area, and thus enabling them to accept more dye. Mercerization also makes the fabric stronger, gives it added luster, and makes it pre-shrunk to some limited extent (Of course this process also leaves a nasty chemical mess to be disposed of).