So some one posted a question to me that I couldn't really answer.. Why are you using the cheapest salt out there with your system? This prompted a discussion about calcium and false readings, nutrient export, ect....
Have you noticed a difference with a more expensive salt? Which would you recommend for a mixed reef? Does this salt type effect your dosing and in what way?
As far as expensive...I think that's relative. What should be in question is value and effectiveness. I think you have to consider the things that make up the cost of salt. If it's an import, you know they are paying a lot to have it brought here...which of course will be reflected in the cost. The added cost in this case doesn't help it score well in terms of value or effectiveness though.
As far as cheap, a salt made here in Ohio, Pennsylvania or Maryland doesn't have to be expensive to be made of top notch ingredients and composed with a top notch formulation.
As far as what's best....that's a far more nebulous question. Aside from store-brand salts, I don't think I would question many salts on the market in terms of basic functionality. The formula for basic artificial saltwater is very old and well-known. What you have to question is whether a particular formulation is what you want for your tank or not. Some are very NSW-like (i.e. plain, "low" numbers), others have extremely enriched formulas, many (like Instant Ocean and Reef Crystals) are balanced somewhere in the middle. Any could be "perfect" for your tank, in theory, depending on what your tank is like and how you run it.
I do wish it was easier to compare salts, but facts about how they "should" mix are always lacking....the best we have is the salt-testing post in that other forums chemistry section. It's very much better than nothing, but getting pretty long in the tooth and is based on (lots of) user testing.
My most common recommendation is to start with Instant Ocean, or whatever basic salt your local shop carries. (Being able to get some on short notice is perhaps the best value a salt mix can have.

) Upgrade to Reef Crystals if you decide to keep stony corals and when the time is right. (As needed.)
$0.02
-Matt