depreciation will be killer on a GM vs a BMW or a Honda. So long term you might actually get all that extra money back that you paid extra for the Honda or BMW.
Then there is reliability, I know the Honda will be rock solid so maintence would not be a factor, the Saturn turbo engine is unproven, and if it was a German made BMW I think it would be better than a USA made Z4 if the Z3 is any indication.
Compare insurance, which will be cheaper, I know the Turbo always gets the insurance people up in arms, but so does the higher price tag of the other 2, so it seems like a wash. The S2000 is a popular target among theives, making rates higher.
The RL gets better gas milage.
The other 2 cars get more gears.
Trunk space goes to the other 2.
It all comes down to how many trips you have to make to the dealer to get something fixed, we won't know that for a few years, but edmunds rates the cars like this for reliability:
S2000 gets 5/5 on mechanic reliability
Z4 gets a 5/5 on "initial quality" <--- pretty worthless 90 days of ownership, I want to know if it will last 2=3 years out:
As far as GM turbo I could compare the Sky RL to, there really isn't anything since its all new, the cloest I could find would be GM's SAAB 9-3, Turbo.
It did very poorly for mechanical reliability at 2/5
So I guess we can hope the Sky turns out to be Honda bullet proof, that is pretty much what you are paying that extra $5000 for. If GM would do a 60,000 mile warranty like Lexus, or a 100,000 like Hyndaui it would go a long way towards easing people back into domestics.