Florence Knoll legit check  

Photos are small and you didn't post dimensions, but it looks legit to me. If it's new enough, it will have a signature:

Thanks for that, the seller told me that this is and older version and that it does not have the signature on the legs.
What does that mean?

The older ones did not have signatures on their legs.
I don't know what the tell-tale signs of a real vs fake knoll sofa are, but recommend you inspect the sofa taking very precise measurement of the frame, photographing the undersides and backsides of everything, the stitching, the fasteners putting everything together, the feet, etc. And before you go, research enough to know if the metal frame should be chrome plated steel vs stainless, and learn how to tell the difference.
If you do all that and compare everything to a known authentic, and of the same rough era sofa you will be able to answer the question for yourself. Or post back here and someone will know.

Thanks for that Leif, If it is an authentic Knoll Sofa, about how much should I expect the price to be around?

No idea. I would buy it for under $10.