Right now, I believe my after-tax stipend is around $1600. As far as getting it back, I believe I get everything back, but I also get to claim a child tax credit, so it may not be the same for you.
I've also wondered about why the 4-year kids go to COT before school. I'm staring my 4th year of school, and I've forgotten a ton from COT. Another aspect of it that was very disappointing is that while at COT, those people who are not going directly into active duty (ie...the people who will be going to school and, thus, on reserve duty) don't go to all the same lectures as the active duty folks. You will go to lectures about being reservists that don't really help or pertain to you at all, while they go to lectures on aspects of active duty, like moving from base to base, etc. Being prior military, it's probably not as big a deal for you, but having no prior experience with it, I was very disappointed. I feel like I can put on a uniform (although, since the BDU's have changed, that's not necessarily true) and I can probably get through a march, but there's a lot I wasn't taught or don't remember.