To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Danitha Capashen's last ability)
However, if you want to complete the set, it can be a bit of a trek
Collect and assemble all 9 to create a cool artwork
The point is to make Gandalf huge and swing with him, and it doesn't really matter how
They all offer a reasonable buff, powerful protection, and great combat damage triggers
When using the Shift 5 ability, which allows the six-cost card to be played one turn early if a cheaper Merlin can be commandeered for the cause, Merlin gets to make use of another feature called Overdeveloped Brain