The G-Rex 4000D is actually the cheapest busboard I can get today. I don't know what's the price of Prometheus but it's sure cheaper than Mediator 4000Di. The G-Rex however is a used unit but it seems to be a minor drawback since I'll get a 6 month warranty along with it.
So... is it worth that 150 Euro? That's 25 Euro less than a new Mediator...