Yes, except if they invent something to make assembling easier (for example a click system of some kind). If prefab elements get so small that a person or a small robot could carry it and so that it could be stacked easily, transported in containers, raised up with elevators etc. it could be cheaper.
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Assembly costs are already a major expense in large buildings. TO drive costs down, the prefab elements will be getting larger, not smaller.
Yes, except if they invent something to make assembling easier (for example a click system of some kind). If prefab elements get so small that a person or a small robot could carry it and so that it could be stacked easily, transported in containers, raised up with elevators etc. it could be cheaper.
Even then, to drive costs down further, you could make them larger and you have fewer of the easier operations.
The minima for cost will always be, make the elements as large as feasible.