Here's what I found out:
Toyota Dealership: $100 to $140
Goodyear Auto: $92
Auto Zone: Free
Can you guess which one we went to? It took about 5 minutes (no appointment) and they were able to give us the codes detecting the problem. Turned out that it was that the gas cap either wasn't on tight enough or is starting to rust a little causing the light to go on. Sure glad we didn't pay around $100 to find that out.