There is no set price. I'll be the devils advocate here ^ and say that buying one that has been on display for a while means the machine is a good one, has been running fine while people play with it, so it is going to last. I would never, ever buy a refurb, why did it need to be refurbed? What was wrong with it in the first place that it needed to be refurbed? When you open something new out of the box, how many times with the crappy quality control of all these places in China, how many times has something not worked right out of the box? A lot in my experience, and it is only getting worse.
I would say if the machine has not had any cosmetic damage, then it should be fine, and the discount should reflect that, and what, if any of the parts in the box it came in are missing, and how will the store warranty it? Consider those things, then you it's up to you and the seller to agree on a price.