Depends on if you want games, or a RGH system, or if it's just the console.

• Just the console - Between 25-40$ is fair based on the condition of the system.
• With games - Between 50-100$ is fair based on number of games and if comes with kinect.
• RGH system - Between 50-200$ is based on the amount of stuff on their hard drives, such games and emulators and more.

But that's just me though.
(Also, this is based on our FB market place prices, also, am in canada.. so might not be general same in your area, but regardless, i think these are fair prices.