There was several incarnations of the 6000d, over the years, so you'd need to be specific about which particular one, in the line-up of evolution, you're referring to. The orignal 6000d was introduced in 1978 I believe. That one is sort of a dinasour by today's standards (but was ground-breaking at the time). It was a fast sweep machine, prone to masking, poor on low conductors, and not very deep (by today's standards, but at the time, deep when compared to TR disc.). If you have the original, you'd be hard-pressed to get $40 for it. But hey, ebay is a fickle thing, and sometimes people pay ridiculous prices.
Then there were some interum stages of 6000's. The last one: 6000Di pro, was the best. It is still a popular model, and could probably fetch more on ebay. If that last one (black box, 950 coil, etc...) is what you have, perhaps $200-ish? I dunno. check finished auctions, look for your exact type, and see what they fetched.