Pracantatio seems really rather pathetically easy to get in comparison to the power these blocks allow... Also, salis mundus really just makes sense for this kinda thing, according to the in-game lore.
Also, I was thinking that a single infusion should usually not consume a full salis mundus, where right-clicking the mundus into one of the blocks fills an internal 'tank' which is drained from during infusion. The salis cost of any single infusion would ideally be calculated based on a combination of the disparity of the aspects, (For example, combining ignis and gelum into a block would logically require a lot of magical coaxing, while combining saxum and terra would be easy) the amount of aspects going into the block, and the effects themselves. (A furnace that burns forever, for instance, should require a decent amount of salis for balance reasons)