It's actually much more cost effective than the usual method.
In the scenario where you membrane first and screed second, using Efflock negates the need for a secondary membrane, and provides better protection. Efflock is designed for tile beds, and cement-based tile adhesives and grouts, providing complete protection for all 3 layers that cause leaching.
1 litre is enough additive for 16m2, or 3 typical-sized bathrooms.
A typical rate for secondary membranes in the Sydney region is $30-40/m2 installed. By comparison, Efflock costs as little as $4/m2* to impregnate a 40mm thick screed, tile adhesive and grout. And remember, a secondary membrane serves no protection for adhesive or grout.