iPhone: $200 + Cost of 2 year contract with broken ATT network.
Griffin OBDII adaptor proprietary for iPhone: $89.99
Works with Windows: NO
Works with IPAD: NO
Works with anything but iPhone: NO

Total: $289.99 + tax plus cost of contract on a crappy network.

Droid Incredible: Free + Cost of of 2 year contract on an ubiquitous network.
OBDII Bluetooth Adapter: $20-35 on ebay.
Works with Windows: YES Some software is free
Works with Ipad: YES Some software is free
Works with any non-proprietary software: YES
Works with Android: Yes actually the best software is already free.

Total: $20-35

Do the math.