Hi there,
I'm not sure about the cost on a new one, but I'm fairly sure that a used one will go for anywhere between £275k for a good 500d, and £375k upwards for a decent 500e. They're also few and far between.
I monitor the boards for used ones just to keep pace with the market and I don't see them come up that often. I think the article in the HCGB magazine typifies the lengths you might have to go to, to get a decent one.
I guess like anything it's all in the amount of effort you put in and then buying it right in the first place. I would also expect to spend money on anything you buy to bring it up to standard.
Joel