The charge per oz would depend on the cost of your ingredients, among other things. I have never sold a salt scrub, so I couldn't tell you what sells best
It's something you need to calculate for yourself. Only for the last part below are others' experiences relevant, but only in the context of where you are selling, and of the same type of image your products represent, etc.:
YOUR ingredient costs, including shipping.
YOUR overhead costs
The profit margin YOU feel is appropriate.
Whatever YOUR market will bear.