IMO your still doing it well wrong.
1. I personally think your not factoring in enough deprecation of the car.
2. The 350z isn't really a weekend car its not bad on fuel.
3. Buy a 350z with 10k to spend on fuel
4. If getting a weekend only car your in the same boat of it sitting there losing money
5. Get a special car for the weekend like a supercharged Z, evo, lotus or TVR etc IMO.
But its your money and whatever makes you happy :-)