Welp-

1st off, remember that EC is relative- some forms of N are more conductive than other forms of N,etc...

There are also matters of dilution- fer instance, using the Advanced SG two part, 5/ml a gallon of each will give you an EC of right about 1.0, while doing the same volume dose w/ each part of the Canna will, as I remember, put you right around .7. ( Base nutes, no addys).

And 10/ml of, say, Cornicopia will put you at about .8 , but this will fry the living hell out of everything. But I've pushed Pure Blend Pro up to like 2.1, without any problems.

It's a relative measurement- it really just tells you how well a solution conducts electricity. From there you can measure the relative concentration of a solution, but it really only compares cleanly w/ a more or less dilute volume of the same solution.

But yeah, there's a "cost of nutrient over time" factor- sometimes the stuff that costs a lot for a little bottle is cheaper long term than the stuff that costs a little for a gallon jug. Depends on how fast ya use it up.