The price of any used keyboard depends on your location, and many other factors peculiar to your area.
When you ask for a good price - that is too vague. It's really not appropriate to place on a web space.
For example now that the Kn6000 is weeks away from release - the Kn5000 is being heavily discounted by dealers selling new Kn5000 keyboards with complete warranties.
The dealers will probably sell these new Kn5000's for about 40% of the list price.
But then your keyboard is worth probably 60% of that price - assuming it is in perfect condition - and updated with version 10, since you are not a dealer - and you cannot offer a dealers warranty.
So - if you expect to get numbers from anyone as to the value of your keyboard - I doubt it very much.
It's a buyers market - and you should describe your keyboard - what accessories go with it - where you are located - what voltage it works on - when you bought it - is it updated - and ask potential buyers to write to you or call you - if they are local.
Your keyboard is only worth - what someone is willing to pay.
If you wish to send me a private E mail - and tell me what you are asking - where you are located - and all other pertinent data - perhaps I can help.