An Ogre's immunity reverse engineers as follows.
--Base Refresh Cost for Physical Immunity is -8.
--Having a Toughness power that only applies to one thing provides a +2 bonus to the Catch. Refresh cost becomes -6.
--The "Catch" in this case is everything the ogre is not immune to. This means it falls under the "not a rare material or attack" type of catch, for +2 Catch again. Generally speaking, if you are only immune to one thing, this is going to be where your Catch is, as a minimum reduction.
--Finally, knowing what not to attack an Ogre with is neither in the common sphere of folkloric knowledge, to the degree that garlic for vampires or similar things would be. However, it is certainly something you can find out without having ever seen an Ogre, much less fought one, so it is neither a +2 or a +0 for research purposes. Therefore, add +1 to the Catch, for a total of +5.
Therefore, an Ogre pays three refresh (8-5) to blow off magic, which, notably, is the same cost as Evocation in the first place. Given that Evocation is useful against a far wider variety of targets than an immunity to magic, one might even be able to argue that the immunity is too expensive. But that is neither here nor there.