Hi,
First, not as a reply to you, original poster, but as a general comment to US readers: in the UK, we have a sliding scale of annual tax on cars that is based on a figure purported to represent the mass of carbon dioxide the car emits per km driven. It seems that the Z4M attracts a higher rate than the 3.0, and I think it is roughly the equivalent of US$820 for the M and $550 for the 3.0 a year. Which, let's face it, is peanuts!
Now, we have fuel consumption. In the UK, petrol is about $2.20 per litre. Frankly, I have no idea what this is per gallon, but I think it is more than you pay. And that is all to the good, for our society, if not for us petrolheads, personally! So, fuel consumption matters to our wallets.
Now, back on-topic... I went through this a little while ago, before I bought my own car. It is far from clear-cut. The M is certainly going to be more to buy, to service, to equip with tyres and to fuel. Probably MUCH more. It also costs more to insure. For driving, it depends what you are after. Even as a very experienced driver of fast sports cars, I think that there are very few opportunities that the maximum performance difference between a 3.0 sport and a Z4M would count for much. Almost all of the time, the ride of the M would count against it. I happen to think that the 3.0 is a well-balanced car, but the M always just takes second place to the Cayman-S.
I assume you live in Scotland. If you are in the Highlands, I can see the attraction of an M-car. (My own experience in an E28 M535i on Rannoch Moor has yet to be bettered.) But the roads in the Central Belt seem to be so congested and ill-maintained, I think I might buy a Citroen DS if I lived there. (I am Scottish, and visit regularly, bit live in North Wales, now.)
The M is probably going to depreciate less than a 3.0 in percentage terms, but what I decided to do was buy a 53-plate 3.0 sport because it seemed to be the car that had done the most depreciation in capital value already, and capital depreciation can trounce all of those running costs, if it goes against you. Compared with the lesser-engined cars, the 3.0s seem to plummet in value over the first couple of years, and then level out, so that with a 3+-year-old car, there is not much difference in value between a 3.0 car, a 2.5 car and a four-cylinder thingy.
So, I probably the answer is that is you do not *really know* you want a Z4M, then don't buy one! There is no sensible reason to do so. The 3.0 is a really punchy car, with a little but of extra comfort that will make you feel you want to use it all the time. OTOH, if you want the ultimate, buy a slightly higher mileage, slightly older car, and the total cost of ownership could be much the same.
Above all, if you can locate a non-franchised BMW expect, you will be able to get good advice and save a fortune without a drop in quality of service. There is a network of people who underwrite trade-ins from BMW dealers. What they do is agree to take a car from the dealer that is in absolutely prime condition, but just too old for the main dealer to handle. These people sell excellent cars at good prices (I have bought several). My local expert got me an excellent (not immaculate), one-owner, full BMWSH 53-plate 3.0Z4 sport with 43k miles for £14.6k, and it has just been serviced (10,9000 to an I2) with 9 months tax and MOT, traded in to a BMW dealer in Essex, and sold though the sub-main-dealer network to me. Everyone is happy!
Hth
Alistair
