Like you said, it all depends how much you work in a month, but my understanding is you get roughly 1 drill period for a days work, which equates to $193 per drill for O-4 over 10 in 2008 before taxes + 1/30 of your flight pay too. If you go on orders longer than 30 days, then you get the active duty benefits of BAH and BAS. You don't have to fly to get paid, that is the most common duty to accomplish as a part timer. I'm not in the Guard yet, but that's what I've learned.

2008 Reserve Drill Pay Calculator - Military Benefits - Military.com