Minimum to my knowledge has always been there, the majority we use is over 3mm average and up to and over 50mm (usually in 2 applications).
We tend to find alot of the products intended for thicker application have a lot grittier body to them, i suppose to strengthen and bulk them up, ths grit is difficult to feather out to nothing and its not smooth enough.
Also with the introduction of fibres in many products this tends to have the same effect, and tends to roll off the trowel and leave a ragged edge which needs rubbing down when dry.
Also when working out the cost per Kg also remember to look at the cosumption rates, i have seen as little as 1.6Kg / mm / m2 and as much as 2 Kg / mm / m2 over the years.
the best way to cost out is look at the price per mm / m2.