I don't think it is a question of cost really, but more of characteristics of the cars...
The 3.0si is a car that is built to satisfy the general market: It is fast, easy to drive, has a good soundtrack, handles perfectly fine, is exploitable and other than the runflats has no bad points really. It is a cracking car. But as I said before, it follows what most people want, which is a sanitized, easily exploitable safe car.
The M version seems to be made with less compromises. It harder, more difficult to drive, more difficult to exploit and needs some work from the driver. You need to be coming back for more and trying a bit harder. It does have more power and does have a louder, less civilized sountrack and does feel special.
As Shunt said, horses for courses. When I had driven the 3.0si I loved it and placed an order for one. When I drove the M I liked it and because it is a car for play only (I don't drive it for work) it made sense for me. I'm happy with my choice, but I'm sure I would've been happy with the 3.0si as well (as soon as I got rid of the runflats).