Sandstorm, it's level 5.
Wall of Salt is a level lower, and nets you a fairly decent amount of salt, as well. Each 5 foot section of the wall yields 4.25 cubic feet of solid salt. 132 lbs of solid salt make up 1 cubic foot, or 72 lbs if its loosely packed. At 5gp/lb, and 7 sections of wall at level 7 when you can first cast it, you are still looking at somewhere around, what, 20k gp/cast? Not as efficient as flesh to salt on a cow, but it comes earlier as a spell, and cheaper as a scroll, with no save to contend with.
Edit; those weight numbers for salt, btw, came from the Salt Institute. Yes, it exists. Their web address is the same as their name, saltinstitute.com