When you say that you "do not think the level of bias from NBC or ABC is anywhere near that of Fox news", you are stating that from your opinion that you are centric. If you do not remove yourself from the equation, and view from the point of a centrist, then your understanding of the impact will be skewed. The better argument is that you do not know the native political leanings of the individual viewers, so the amount of impact would vary per viewer, and make calculations difficult.
If you take the overall media, it is recognized as having a left of center bias, and journalists are overwhelming liberals/Democrats. In advertising, one of the main methods to establishing a narrative is repetition, which we see broadcast in mass from the liberal media. FNC stands alone on the conservative side. You will have extreme believers on both sides of the political aisle watching their favorite stations for certain, but to discount the overall amount of bias broadcast on a daily basis is disingenuous.