Per Erika Esola:

This information is coming from a source close within Chris Johnson's camp. John Idzik's number of $3-4M is pretty much what I've been saying is appropriate, although I would personally like to keep it under $3.5M per year. For all of you that have said Idzik is low-balling Johnson, I think this proves that's false. The current market rate for a starting running back is roughly $3M per year. Idzik is offering slightly more than that, and Johnson is seeking a good chunk more.

As Esola originally noted, Johnson is planning on checking out other teams, such as the New York Giants. I sincerely doubt he gets $5M/per from another team, but we'll see. How much per year are you willing to go up to get Johnson?

EDIT: Kristian Dyer has confirmed that Idzik offered a multi-year deal, we're looking at $3-4M per, as Esola said.