I'm seeing prices from $150-$250. Going rate seems to be $175-$200. That is where I would start. One thing you have to keep in mind is you can get a DX7s for about $250 with free shipping.
YOu can try to sell them together, but the DSMX will work really well on the DX8. And since you already have it, and you KNOW you will have another aircraft soon, you will have it to put it in. With that in mind, I would keep it and just sell the Tx by it's self. Market it as "Perfect for BnF.