Well lets see, Im a technician at a hyundai dealership, and i actually worked on a car before where some genious installed a optima battery backwards as you just quesioned. Bellow is a list of what i had to fix/replace:
1. Underhood Fuses and Fuse Box
2. Main engine harness
3. Underdash/Cabin SRS Harness
4. Electronic Satellite Airbag Sensors
5. One Negative battery Cable.
Total - $2840 Plus labor
It all depends for how long you leave the battery connected that way. He apparantly left it on until everything was fried and the cable on the battery melted away. If you leave it on for a second or two you may just pop some fuses, either way, it will cause some damage.