Dude that sucks! I haven't heard of any transmissions going out. I'm going to guess $1000-$1500, but just a guess. Sorry I can't help much.
Transmission is toast. 3rd and Reverse are completely gone. Dealership wanted north of $4k to replace it, and my local shop was over $3k b/c of the lack of replacement parts that are out there.
After working with the dealership they're now covering 70% of the certified GM transmission, and it also has a new 3yr/100,000 mile warranty.