Few things...

- Full leather interior over the M3.
- Faster then the M3.
- Newer than the M3.
- Its a convertible (always priced higher then hard tops)

I'd retry this comparison between a Z4 Coupe and an M3 Coupe. The prices should be a lot closer. Or look at the price of an M3 Convertible...

Apparently a Z4 M roadster is still a bargain compared to a Boxster S (perhaps not when fully loaded...)...