You miss my point.
An E39 M5 is an expensive car to own at any point - more so now they are getting increasingly older, more fussy and harder to find in excellent condition. Even for a low mileage user, it's a big commitment in terms of both time and money. It's a horrendous money sink. However, for some people - it's a money sink that's worth it to own one. I totally get that. I've been tempted enough myself on numerous occasions. But if that's not you then it's just an irritating millstone around your neck. It's not a car you go and view because you need to buy 'some BMW' and one pops up in an Autotrader search. If you buy one in those circumstances I give it a few months before it's back up for sale.
He considered a 730i and now a 330Ci. These are quite different to the M5 both in terms of exposure to cost and suitability for continuing to do things like put perhaps 4k business miles in a single month on it. He's also coming from a Honda Civic diesel, it couldn't be more different. Researching into costs will show you two camps, there are those who think you can reduce some of them by cutting corners etc etc but if you don't wish to keep it the awesome supersedan it once was then why bother.
I get the impression he fancies a nice, big, *and comfortable BMW for wafting about and soaking the miles up in with a bit of added performance, too. That's what a 730i, had it not been a shed, will do. That's what a 330Ci can do, as well. The 330ci especially is capable of doing this without having a ridiculous fuel range and chewing a set of rear tyres in 6000 miles. It's easier to live with, it's easier to own and it's less likely to be a constant source of money-sapping annoyance.
With the age they are now an E39 M5 is an enthusiasts car only. If it's the right car for you, you know it's the right car for you and nothing else compares. You wouldn't even have gone to look at a 730i or think about a 330ci.
Personally, from reading this thread, I think a 330i or 530i of some description is probably the ideal answer to this thread. They are a fantastic blend of everything really - ok sure neither is going to be super cheap to run nor are they going to be as hassle free as a nearly new Civic but then that isn't what you are expecting anyway. The 330Ci is involving to drive, has a range of bodystyles inc coupe and cab, the 530i is beautifully built, soaks up the miles effortlessly and still enjoys a decent A road. The 330CI is 'Sport with added waft', the 530i is 'waft with added Sport' if you see what I mean. They'll even do mid 30's on a run mpg wise, which is useful if nothing else for the fuel tank range alone!
Assuming 12k or so a year, the above would be my opinion of the best way to go about this.