Hello,
I would think that anywhere in the $6000-$7000 range would be a fair price. I have seen refurbished Mix Plus systems selling at Guitar Centers at around $5000, but Guitar Centers like to price things high. Hope that helps any, and if you are going to sell a system, drop me an email cause I am looking to buy one in the future when I have the money.
EyeSound

Thanks EyeSound. That's actually what I thought it might be worth as well. We are looking to sell very soon, I will let you know.