Although the 525d and 530d seem ridiculously dearer than the 520d, there will (eventually) be a lot more discount on them. Though funnily enough when I was buying the 320d and the cost to change - and my target figure was coming up, the dealer actually had more discount in the five than in the three and this was two years before it was to be replaced.
So, although the 520d price is trimmed to suit the marketplace (520d makes up 80% of Five series sales in the U.K.) they still, at that time, had more wiggle room in the 520 than a 320 - even though the 320 base price was higher!
Surprisingly, it's five series sales which count for BMW - in Britain at least. They provide the biggest profit (you would have thought that would be the E9x) and that explains the price of the bigger engined cars. Besides, that difference in price won't put off the businessmen who have some say in what they drive - they will want the big engine regardless.