The least expensive is a mason mix. Mix the mortar (no lime) so that it will just form a cohesive ball. Use two 1/4 inch rods as screed guides for a minimum depth and a 6 foot level to vary the depth for a level surface. If a feathering compound is desired then Ardex K-15 and SIKAFLEX 2C work well.
A feathering compound, also called a self-leveling compound, is a liquid chemical that is used to level or to flatten a floor. When poured on the floor this liquid dries hard on it making it ready for tiling.
