I have a 2008 chevy aveo that won't shift into gear unless we press the shift lock release. What is causing that and how expensive is it to fix?

It could be a few things....I just had this same problem with my 2008 chevy aveo this week. Also, my check engine light came on the very next day after the gear shifter started locking. I called Bob Bell and they told me that the gear shifter locking is probably due to a sensor going bad that tells your car it is in park. However, after taking my car to my brother-in-law (who has very LITTLe knowledge about repairing cars), we discovered that my brake lights were not working either. He told me that is why my check engine light is on - because it is extremely dangerous to drive without brake lights. We checked the fuses to make sure there was no fuse blown and they were all just fine. So bottom line, the problem was that there is a brake switch under your dash board where your brake is. My switch keeps sticking and will probably eventually need to be replaced. When the switch works properly, my brake lights work fine, my gear shifter unlocks and my check engine light goes out. Not sure how much it is to fix this switch if that is the problem but I hope it helps!