the difference in price is rather a marketing decission than a cost problem.
i think yamaha wanted their arranger to be known as only 61 keys. is again the marketing. this way, anyone who does not necessarily wants 76 or more keys, will think automatically of tyros, and skip the other options.
and there are advantages having a lighter and smaller keyboard.
so, yamaha is just happy with this, and probably none of the tyros series (if continued) or psr series will ever have 76 keys again.
probably when that will happen, there will be a new series released, as a "brand new and exciting keyboard". and that will be probably after T3.