I would prefer to see it priced as a separate app. It it were free or nominally charged, Omni customers who do not use iPhones would effectively be subsidizing those who do.

Developing these things incurs costs, and deflects capacity from other development projects. The costs should be allocated to those who benefit.

My impression, judging by comments on this forum, is that those who do use iPhones are really very keen to get hold of an iPhone version of OF, and that demand will not be highly sensitive to price.